FOOTNOTE:
[4] The elephant, the thigh-bone of which, measuring nearly 3 ft. in length, is drawn in Fig. 32, is a large Indian one. This species is exceeded in size by the African. See "Science from an Easy Chair," Second series, p. 123.—The largest elephant the bones of which are known is the Elephas antiquus of the Pleistocene, bigger than either of the living species and bigger than the mammoth, Elephas primigenius. The arm-bone (humerus) of one of this species (Elephas antiquus) lately dug up near Chatham and now in the Natural History Museum, is 4 ft. 3 in. in length.
CHAPTER VII
WHAT IS MEANT BY "A SPECIES"?
THOSE who take an interest in natural history must find it necessary to know what the naturalist means by "a species" of animal or plant. What does he mean when he says: "This is not the same species as that," or "This is a species closely allied to this other species," or "This is a new species"? What are the "species" concerning the origin of which Darwin propounded his great theory? There is really no English word which can be exactly used in place of the word "species." I often have to use the word when writing about plants or animals, and should like once for all to say what is meant by it. One might suppose that a "kind" is the same thing as a species. And so it often is; but, on the other hand, by the word "kind" we often mean a group including several species. For instance, we say the "cat-kind" or the "daisy-kind," meaning the "cat-like" animals or the "daisy-like" plants. The expression "the cat-kind" includes the common cat and the wild cat, and even leopards, lions, and tigers, each of which is a species of cat. And by the "daisy-kind" we understand a group including several species of daisies, such as the common daisy, the ox-eye daisy, the camomile daisy, the michaelmas daisy, and others. Hence we cannot translate species simply by the word "kind." "Kind" is the same word as "kin"—"a little more than kin and less than kind," runs Hamlet's bitter pun. "Kind" means a group held together by kinship, and it may be a larger or a smaller group held together by a close kinship or by a more distant one. "Sort," again, will not serve our purpose as an English translation of "species." For, although "a sort" implies a certain selection and similarity of the things included in the "sort," the amount of similarity implied may be very great or it may be indefinitely vague and remote. Hence naturalists have to stick to the word "species," and to use it with a clear definition of what they mean by it.
Suppose we get together a large unsorted collection—many hundred "specimens" or individuals—of the common butterflies of England. Then, if we look them over, we shall find that we can pick out and arrange the specimens into definite groups, according to their colour-pattern. We find that the kinds which we readily distinguish are called in English the swallow-tails, the whites, the sulphurs, the clouded yellows, the tortoise-shells, the peacocks, the red admirals, the painted ladies, the gatekeepers, the meadow browns, the heaths, the coppers, and the blues. There might be others in such a collection, but that is enough for our purpose. On examining the specimens closely, we find that the colour-markings and "venation" or network by which the wings are marked and the shape of the wings, body, and legs of all the specimens of the swallow-tails are almost exactly alike, and unlike those of any of the others. We shall find if we have a dozen or two specimens that there is a slight difference in the pattern, size, and colour of wing of some of the swallow-tails, dividing them into two groups, which we soon ascertain to be the males and females; but this is so small a difference that we may ignore it. The swallow-tail is obviously and at once distinguished from any of the other butterflies in the collection by its colour-pattern and shape. So also with the others, there will be many specimens in each case agreeing in colour and pattern, and recognizable and distinguishable from the rest by the colour-pattern and by the "venation" or "nervures" of the wings. If we collect butterflies again in other years and in other parts of the country, we find the same set of shapes and patterns exactly, corresponding to what we have learnt to call swallow-tails, whites, sulphurs, clouded yellows, tortoise-shells, etc. There are, we thus learn, several distinct, unchanging kinds of butterfly, which are common in this country, and appear every year. Similarly we may go into a meadow in spring, and gather a number of flowers, and a naturalist will roughly arrange our bouquet into "kinds"; there will be the buttercups, the daisies, the clovers, the dead nettles, the poppies, the roses, the orchids, etc.
If, now, we look more carefully at our collection of butterflies, sorted out roughly into kinds or species, we shall find that the "whites," although holding together by a close similarity in having merely white wings edged and spotted with black, yet differ amongst themselves, so that we distinguish a larger kind, the large garden-white, and a smaller, commoner kind, the smaller garden-white, and we distinguish also the green-veined white, and possibly the rare Bath white, each of them differing a little in their spots as well as their size. These different sorts of "whites" can, once our attention is drawn to the matter, be readily distinguished from one another, and constantly are found in our collections. We thus arrive at the conclusion that, though the whites are much alike, and are a kind distinct from the other kinds of butterflies, yet the "whites" themselves can be divided into and arranged as several kinds distinct from one another. In fact, we discover (and an illustrated book on butterflies confirms us in the conclusion) that there are several ultimate kinds of whites which cannot be further separated into groups. These are what are called "species." The whites are therefore not a single species, as are our British swallow-tails, but a group of species, closely related to one another. We find the same thing to be true with regard to the blues. Though they are much alike, agreeing in a variety of details of spotting and colour, yet we can distinguish the chalk-hill blue, the common blue, the azure-blue, the Adonis blue, and others, as distinct "species" of blues. Then, again, when we carefully examine our English specimens of tortoise-shells, we find that there are two distinct "species"—the greater and the smaller—differing not only in size, but in pattern; and when we compare with these the painted lady and the peacock and the red admiral, we find that there is a certain agreement of wing-pattern (venation and outline) and details of shape among them all, although their tints and the shape of the spots and bands of colour differ. These different species "hold together" just as the whites do and just as the blues do. Naturalists have met the need for expressing this similarity of a number of distinct species to one another by introducing the term "genus" for such a group. In fact we arrange several species into a "genus." The "genus" is a "kind," but a more comprehensive "kind," than is a species. The species is an assemblage of individuals closely alike to one another; the genus is a group of species which are more like to one another than any of them are to other species.
Naturalists give to every genus a name, and also a name to each species in the genus. Since we naturalists want to know what butterflies or other species of animals and plants are found in other countries, and to be sure that we all (whatever our native language may be) mean the same thing by a name, Latin names are given to the genera and the species, and are necessarily used when one wishes to be sure that one is understood. The greatest trouble is taken to make certain that the name used is applied only to the original species and the original genus to which it was applied, for only so can one be sure that a writer in America or one in Italy or France means the same thing by a name as we do here in England. This is rendered possible and is actually brought about by the preparation of catalogues in which the species are described and figured, especially with regard to obvious points of detail which are constant, and are called "specific characters." These are chosen for special description, not haphazard, but with a view to their being recognized with certainty by those who study other specimens. Another extremely important proceeding in connection with this purpose of uniform naming, which involves vast labour and expense, is the maintenance of great collections of preserved animals and plants by the State in all civilized countries. In these collections either the original specimens to which names were given by recognized describers (called "type-specimens" or "the type") are preserved, or else specimens which have been compared with those original described specimens, and authoritatively ascertained to be the same as the "type." The maintenance of accuracy and agreement in regard to the names of all the "species" of plants and animals is a big task. It is now carried out by international councils, in which the skilled naturalists of the world are represented. Certain principles have been agreed upon as to the method of determining the priority of one name over others which have been employed for one and the same species by naturalists of different countries and at different times, and a general agreement as to what names are to be used has been arrived at. It is a matter which has involved a great deal of uncertainty and dispute, and still causes difficulty. By the exercise of good sense, and in consequence of the existence of an urgent desire really to understand one another, there is now every year an increasing uniformity and agreement among naturalists about the exact name to be applied to every species of living thing.
Returning to our collections of butterflies and meadow flowers, we may take the names of some of the species and genera as an example of the system of naming in use by scientific naturalists. The common swallow-tail is assigned to the genus Papilio. Its "specific name" is "Machaon," given to it by Linnæus, hence it is spoken of as Papilio Machaon. It is found in various parts of Europe as well as in England. But in Central Europe (often seen in Switzerland) there is also another species of swallow-tail, which only occurs as a rare accident in England. This is the pale swallow-tail, differing, not only by its paler colour but by definite spots and markings of the wings, from the English species. Its species name, or "specific name," is "Podalirius," and so it is known as Papilio Podalirius. Species of Papilio are found all over the world; more than 500 are known. Our two commonest whites belong to the genus Pieris—they are called respectively Pieris brassicæ (the larger) and Pieris rapæ (the smaller). The green-veined white is Pieris napi. Each of these three is called after the plant, cabbage, rape, or turnip, on which its caterpillar feeds. The rare Bath white is Pieris daplidice. Its caterpillar feeds on mignonette. There are dozens of species in other parts of the world allied to our "whites," which naturalists have carefully distinguished and characterized by their marks.
Several of our most beautiful species of English butterflies which are much alike have been enrolled in one genus—the genus Vanessa. This genus includes the great tortoise-shell, called Vanessa polychloros; the smaller tortoise-shell, Vanessa urticæ; the peacock, Vanessa Io; the painted lady, Vanessa cardui; the red admiral, Vanessa Atalanta; and the comma butterfly, Vanessa C-album. There are other European, Asiatic, and American species of Vanessa.
In the same way we find with our meadow plants that what we at first thought was a single kind, "the" buttercup really bears a name applicable to a genus in which are several common species. The genus is called Ranunculus, and there are several common English species with yellow flowers, but distinguished from one another by definite characters. They are Ranunculus acris, Ranunculus flammula, Ranunculus bulbosus, Ranunculus arvensis, Ranunculus ficaria (the lesser celandine). And then there is the white-flowered Ranunculus aquatilis—a common pond plant. Clover, again, is by no means the name for a single species. The clovers form the genus Trifolium, and in any English meadow we may come across the white clover, Trifolium repens; the red clover, Trifolium pratense; the hop clover, Trifolium agrarium: the strawberry clover, Trifolium fragiferum; the haresfoot clover, Trifolium arvense. So it is with the plants which at first sight we distinguish merely as "daisies." There are several distinct genera of daisies—Aster, Bellis, Chrysanthemum (ox-eye), Anthemis (camomile), and others, with several distinct species in each genus.
Enough has been said to show the reader that the mere notion of "kinds" does not carry the same meaning as "species," but that there are a number of regularly occurring definite forms of both animals and plants which can be arranged in groups consisting only of individuals which are very nearly identical with one another. A group of living things of this degree of likeness is called "a species," and receives a name. A less degree of likeness holds together a number of species to form what we call a genus, and the name of the genus is cited together with the name of the species when we wish to speak of the species with clearness and certainty. This system of double names we owe to the great Swedish naturalist of the eighteenth century, Linnæus. He proposed also that the relationships of living things to one another should be further expressed by grouping like genera into "families," then like families into "orders," and like orders into "classes." And since his day we go further and group classes into "phyla" or great stems of the animal pedigree. In this way a complete hierarchy or system of less and more comprehensive groups has been established, and is the means by which we indicate the natural groups of the family-trees of plants and of animals, what, in fact, is called the "classification" of each of these great series of living things. Linnæus compared his system of groups to the subdivisions of two armies. Thus, the one army represents the whole animal series, the other the whole vegetable series. An army is divided into (1) "legions," these into (2) "divisions," "divisions" into (3) "regiments," regiments into (4) battalions, and battalions consist of (5) companies, consisting of individual soldiers. According to Linnæus, we may compare the legions to classes, which are divided into orders, comparable to divisions; these into families, comparable to regiments; these into genera, comparable to battalions; and these into species, comparable to companies, or ultimate groups of individual units or soldiers.
Just as the legions, divisions, regiments, battalions and companies of an army have each their own name or at any rate a distinctive numeral assigned to them in order that they may be cited and directed, so are names given to each class, order, family, genus and species of the classification or enumeration of the kinds of animals and plants. Here, for instance, are the names of the greater and smaller groups in which our common "white" finds itself enrolled. Class—Insects. Order—Lepidoptera. Family—Pieridæ. Genus—Pieris. Species—brassicæ.
CHAPTER VIII
MORE ABOUT SPECIES
I WROTE in the last chapter of the recognition of that degree of "likeness" or kinship in animals and plants which we point to by the word "species," and of the grouping of several similar species to form a "genus," and of several genera to form a family, of families to form orders, and of orders to form classes—and of the giving of names to all these groups. Whilst the making of this or that lot of species into a distinct genus, and giving it a new name is a mere matter of convenience for the indication of more or less important agreements and divergences, and is to a large extent arbitrary or an expression of opinion—it has always been recognized among naturalists that the group called "a species" is not a mere convention, but has a real natural limitation. It is true that the actual things which we see in studying natural history are so many units or individuals. But the possibility of arranging these by pattern, colour and shape into ultimate companies of which all the units are alike and differ from all the units of another company, has been regarded as a natural fact of primary importance and not a mere convention or convenience. The conception of the "naturalness" of a species depends really upon a further qualification of great importance as to what we naturalists understand by it.
We find by rearing plants from seed and by causing animals to breed under actual observation that the individuals of a species pair with one another, and not with individuals of other species, and further, that the young which they produce are like the parents—show themselves, in fact, to be of the same "species." The species continually year after year reproduces itself with little variation, though some variation does occur. The faculty of pairing only within the group, of never naturally breeding with members of other groups, has accordingly been adopted as a test of species. Hybrids between two species do not occur, except in very rare cases, in the state of nature. It is not always the case that the members of two species cannot possibly pair together, but it is the fact that they do not do so. Man sometimes brings about such crossing or hybridization, and it is a curious fact that the hybrids are often infertile or give rise only to weakly offspring, which could not survive in the natural struggle for existence. Sometimes, however, when the two hybridized species happen to come from regions of the world remote from one another, the resulting hybrids establish a vigorous race. There are real obstacles (of which I will say more below) in natural conditions to hybrid-breeding between any two species which occur naturally in the same territory. Thus the idea of a species is expanded so as to be not merely "a group of individuals of constant likeness in form and characteristics," but we add to that definition a living or constitutional quality expressed by the words, "which produce fertile offspring by pairing with one another, but do not pair with the members of other species."
This enables us to distinguish the conception of a "species" from that of a "variety" or a "race." We find occasionally peculiarly-marked examples of a species of plant or animal, or even local races of peculiar form; but we do not regard them as "distinct species" if we find that they breed as a rule with the ordinary members of the species. The decisive test is the breeding. If the variety is found not to breed with the regular species, but to keep apart and breed only with other individuals like itself, then we say, "This is no mere variety! It is a distinct species!" Unfortunately we have vast series of animals, insects, and others, from all parts of the world, collected and preserved in our museums, of which we know only the dead preserved specimens. So that we cannot be sure in doubtful cases whether a series of forms differing a little from the ordinary members of a species indicate distinct species, as defined and tested by breeding. We have in such a case to note the difference, and record it either as a variety or as a species by a guess at the probabilities one way or the other. Naturalists really intend by the word "species" to designate a form represented by numerous like individuals, which, in the present natural conditions of the region they inhabit, have attained a certain "stability" of distinctive form and character (not without some variability within definite limits) and constitute a more or less widely distributed population, the members of which inter-breed but do not produce offspring with other allied species.
A good case by which to exhibit further our conception of a species is that afforded by the species which are united in the genus Equus—the horse-genus. There are living at the present day several wild kinds of Equus—namely, the wild horse, or Tarpan, of the Gobi desert of Mongolia, called after the Russian explorer Przewalski; two kinds of Asiatic wild ass, called the Kiang and the Onegar; the African wild ass, and two or three kinds of zebra. There are, besides, many kinds of domesticated horses, ranging from the Shetland pony to the Flemish dray horse, and from the Shire horse to the Arab. Then there are many kinds of fossil extinct horses known, some of which clearly must be placed in the genus Equus with the living kinds; others which have to be separated into special genera (Hippidium, Onohippidium, etc.). Now, as to the living forms or form-kinds of the genus Equus—which are we to regard as true species, and which are only varieties and races of lower significance than species? The answer is clear enough in regard to several of them. The wild Mongolian horse and all the domesticated horses are varieties, races, or breeds of one species, judged not only by such marks as the possession of callosities on both the hind and the fore legs, but also by the test of breeding. They breed together and produce persisting races. But the asses and the zebras, though they will form mules with the horse, do not in a state of nature freely breed with it. When an ass or zebra is mated by man with the horse it will produce hybrids, called "Mules," but will not in "a state of nature" establish a hybrid race. The asses and the zebras are distinct from the horse, not only in markings and certain details of shape and hair, but in the fact that they cannot be fused into one race with him. There are no sufficient experiments on the aloofness of zebras and asses from one another in regard to breeding, although it seems that they cannot establish a mixed race, and are, therefore, distinct species judged by that test as well as by their form and marking. It is not known whether the so-called species of wild ass—the Asiatic and the African—would prove to produce fertile or infertile mules if intercrossed, nor has the test been applied to the very differently-marked local races of the African zebras—Grevy's zebra, Burchell's zebra, and the mountain zebra. It is likely enough that the three or more "species" distinguished among zebras on account of their being differently striped, and existing in different localities, would be found to breed freely together, and prove themselves thus to be entitled to be regarded as local "varieties" or "races," but not as fully-separated true species.
Thus one sees how difficult it is to have knowledge of the breeding test, even in regard to large animals. It is obvious that the difficulty of obtaining it in regard to the thousands of kinds of minute creatures is much greater. Yet when they say, "This is a distinct species," naturalists do mean that it is not only marked off from other animals or plants most like to it by a certain shape, colour, or other quality or qualities, but that it breeds apart with its own kind and does not naturally hybridize with those other forms most like to it.
Although the kind of naturalist called a "systematist" who makes it his business to accurately describe and record and distinguish from one another all the existing species of some one group—say, of antelopes, of mice, of flowering plants, of fishes, or of fleas—has only a knowledge in a few instances of the breeding of the organisms which he describes as "distinct species," he yet does know, in regard to some one or more of his species in most groups, the facts of pairing and reproduction, and what are the limits of variation in the markings and other characteristics of at least one or two species definitely submitted to the "breeding test," that is to say, ascertained to be "true physiological species," kept apart by deep-seated chemical differences in their blood and tissues. Hence it is legitimate for him, by careful balancing and consideration of all the facts, to determine—not absolutely, but by analogy—the value to be assigned (whether as indicating true species or merely varieties capable of pairing with the main stock) to points of difference among the specimens of a dead collection brought from some distant land or from some position in which it would be impossible to make observations with regard to "pairing" and "breeding true."
Some 400 species of fleas have been described, and we are certain as to the value of the characters relied on to distinguish those species, owing to what we know of the breeding of some common species of fleas. The flea of the domestic fowl, that of the domestic pigeon, that from the house-martin, and that from the sand-martin—used to be considered as one species until they were carefully examined twenty years ago. In reality each of them has its own peculiar "marks," and they do not mix with one another. The nests of the sand-martin yield only one species of flea, namely that peculiar to the sand-martin. The hen-house, the dove-cote, and the nests of the house-martin yield each their flea maggots, which can be reared and become in each case a distinct species with definite recognizable "characters." On the other hand, the flea of the rabbit gives an opportunity of studying the limits of variation in a "good" species. Rabbit warrens swarm with the rabbit flea, and often a great number are found on one rabbit, the individual fleas "varying"—"differing" from one another to a slight extent. The "systematist" thus gets to know what organs are variable within the limits of an undoubted physiological species of flea, and what are comparatively constant—so that he can form a reasonable opinion about the claim of other specimens which he may receive without full history of their habits, to be regarded as true distinct species.
The fact that most important chemical differences of the blood and digestive juices often accompany the small external differences which enable us to distinguish one species of animal or plant from another, makes it obvious that the knowledge of species is a very valuable and necessary thing. One species of flea, the Pulex Cheopis, habitually carries the plague bacillus from animals to man, and is a cause of death; other species, extremely like it in appearance, but distinguishable by a trained observer, do not carry the plague bacillus, but if they swallow it, destroy it by digestion. One species of gnat, the common grey gnat, digests and destroys malaria germs when it sucks them up with blood; in an allied species, the spot-winged gnat or Anopheles, the chemical juices of the gut allow the germ to live in it and multiply, and so to be carried to men by the gnat's bite. So with many other flies and parasites the recognition of the dangerous species is of vital importance, and that recognition often depends on minute features of form and colour not at once obvious to an ordinary observer.
But this recognition of distinct species is, from the point of view of the study of Nature, only a preliminary to the question, "How did these species come about? How is it that there are so many species, some very like one another, forming genera, and these genera grouped into related families, these into larger groups, and so on, like the branches of a family tree?" The answer to these questions given by Linnæus was: "There are just so many species as the Infinite Being created at the beginning of things, and they have continued to propagate themselves unchanged ever since." The answer which we give to-day is that the appearance of a huge family tree which our classification of animals takes is due to the simple fact that it really is neither more nor less than a family tree or pedigree—the "tree of life," of which the green leaves and buds are the existing species. Further, we hold that the existing species of a genus have "come into existence" by natural birth from one ancestral species, its offspring having slightly varied (we are all familiar with this individual variation in our own species, in dogs, cats, trees, and shrubs), and that the varieties have wandered apart and become continuously emphasized and selected for survival by their fitness or suitability to the changed conditions around each of them. Meanwhile a natural destruction, or failure of intermediate forms to survive, has gone on.
CHAPTER IX
SPECIES IN THE MAKING
A SERIES of important conceptions are implied in the word "species," as used by naturalists. Some of these we have noted in the last chapter. There is first, as a starting-point, the conception that a species is a number or company of individuals, all closely and clearly alike (though presenting some minor individual variations), and capable of sharp separation by certain "characters" from other similar groups or companies. Then follows the addition (2) that the species is constant if the conditions of life are not changed, or but little changed, and that year after year it reproduces itself without change. It has a certain stability (but not permanent immutability) greater in some species than in others. Next we find (3) that the species constitutes a group of individuals which have descended by natural breeding from common parents, not differing greatly from the present individuals. They are, in fact, one "stock." Then (4) that the species is a group, the individuals of which pair with one another in breeding, but do not pair with the individuals of another species, and that this is due to various peculiar and inherent chemical, physiological and (in higher animals) psychological characteristics of the species.
We have now further to note that species have their special geographical centres of origin from which most spread only a small distance, whilst others have a wonderful power of dispersal, and have become cosmopolitan. Moreover, we find that some species are numerically very abundant, others very rare; that rare and abundant species have often invaded each other's territory, and exist side by side.
Whilst we often find a number of species, fifty or more, so much alike that we unite them in a single genus (as, for instance, in the case of the cats, lions, tigers, leopards, which form the genus "Felis," and the hundred or more species of the hedge brambles or blackberries, which form the genus "Rubus"), there are many species which to-day have, as it were, lost all their relatives and stand alone, the solitary species in a well-marked genus, or have perhaps only one other living co-species. And sometimes (curiously enough) that one co-species is an inhabitant of a region very remote from that inhabited by the other. Thus the two living mammals called tapirs (genus Tapirus) inhabit, the one the Malay region, and the other Central America. This is explained by the fact that tapirs formerly existed all over the land-surfaces of North Europe, North Asia, and North America, which connect these widely-separate spots. We find the bones and teeth of the extinct tapirs embedded in the Tertiary deposits of the connecting regions.
Once we have gained the fundamental conceptions as to what is meant by a "species," we are able intelligently to consider innumerable facts of the most diverse kind as to their peculiar structure and colours, their number, localities, their interaction and dependence on other living things, their modifications for special modes of life, their isolation or their ubiquity. We can discuss their genetic relations to one another, and to extinct fossil species, which have all been to a very large extent "accounted for" or "explained" by Mr. Darwin's theory of the origin of species by the natural selection of favoured races in the struggle for existence. But there is always more to be made out—difficulties to be removed, new instances to be studied. The classification of the genera of plants and animals, with their included species into larger groups, helps us to state and to remember their actual build and structure, and to survey, as it were, the living world, from the animalcule to the man, or from the microbe to the magnolia tree. Every one interested in natural history should carry in his mind as complete a scheme of the classification of animals and plants as possible.
The older naturalists held that species were suddenly "created" as they exist, and have propagated their like ever since. Darwin has taught us that the present "species" have developed by a slow process of transformation from preceding species, and these from other predecessors, and so on to the remotest geologic ages and the dawn of life. The agents at work have been "variation"—that is to say, the response to the never-ceasing variation of the surrounding world or environment—and the survival in the struggle for existence of the fittest varieties so produced.
There is nothing surprising or extraordinary in the existence of variation. The conditions of life and growth are never absolutely identical in two individuals, and the wonder is not that species vary, but that they vary so little. The living substance of animals and plants is an extremely complex chemical substance, ever decomposing and ever being renewed. It is the most "labile" as it is by far the most elaborately built-up chemical body which chemists have ever ventured to imagine. It differs, chemically, not only in every species but in every individual and is incessantly acted upon—influenced as we may say—by the ever-changing physical and chemical conditions around it. At the same time it has, subject to the permanence of essential conditions, a definite stability and limitation to its change or variation in response to variations of its environment. That part of the living substance which in all but the lowest plants and animals is set aside during growth to form the eggs and sperms by which they multiply or "reproduce" themselves, is called the "germ-plasm," and is peculiarly sensitive to variations in (that is a change in) the environment of the plant or animal.
New conditions of life (locality and climate)—unusual food or reproductive activity—act often in a powerful way upon the germ-plasm and cause it to vary—that is to say, they alter some of its qualities, though not necessarily disturbing in any way the general living substance of the organism so far as to produce any important change perceptible to the human eye. In consequence, the young produced after such disturbance of the germ-plasm are found to differ more from their parents than in cases where no such disturbance has been set up by the natural never-ceasing variation of the surrounding world. This fact is well known to horticulturists and breeders, and is made use of by them. When a gardener wishes to obtain variations of a plant from which to select and establish a new breed, he deliberately sets to work to disturb—to shake up, to act upon in a tentative, experimental way—the germ-plasm of one or more parent plants by change of soil, climate, food and often by cross-fertilizing them with another breed or variety. In this way he to some extent "breaks" the constitutional stability of the germ-plasm of the plant and obtains abundant "variations" in the offspring. These are not precisely foreseen, and show themselves in all parts of the new generation. But some of them are what the gardener wants, and are "selected" by him for retention, rearing and breeding.
The response of the germ-plasm of organisms to the stimulus of new environmental conditions has been compared to that of the well-known pattern-producing toy—the kaleidoscope. The bits of glass, beads and silk which you see in a kaleidoscope, forming by reflection in its mirrors a beautiful and definite pattern, are changed by a simple vibration caused by tapping the instrument into a very different pattern, the coloured fragments being displaced and rearranged. The apparent change or variation is very great though produced by slight mechanical disturbance, and the new pattern is altogether without any special significance—the fortuitous outcome of a small displacement of the constituent coloured fragments. We can imagine that similarly slight disturbances of the organic molecules of the germ-plasm may produce considerable and important variations in it and the new growth to which it gives rise: and, further, that these variations may prove to be either (1) injurious, or (2) of life-saving value, or often enough (3) of no consequence whatever although bulking largely in our human eyes and thereby misleading our judgment of them.
There is no reason to doubt that the same sequence of events occurs in nature apart from man's interference. Changes occur in the earth's surface, or the organism is transported by currents of water or air into new conditions. The germ-plasm is "disturbed," "shaken" or "shocked" by those new conditions, and a variation, in several structures and qualities of the offspring subsequently produced, follows. Then also follows the selection of one of the new varieties by survival of the fitter to the new conditions into which the organism has been transported or have developed in the region where it was previously established.
This process of germ-variation is obviously as necessary and constant a feature of the living organism as is the variation in the contour of land and sea and in the extent of the polar ice-cap—a necessary feature of the physical conditions of the terrestrial globe. But it is the fashion with a certain school of writers nowadays to declare that "variation" in organisms is a "mystery" unsolved. Another very common and almost universal error is to overlook the fact that variation is constitutional and affects whole systems of organs and their deeply related parts, and is not, as it is so frequently and erroneously assumed to be, a mere local affair of patches and scraps visible on this or that part of the surface of an animal or plant. These superficial "marks," readily seen and noted by the collector, are rarely of any life-saving importance: they are but the outward and visible signs of deep-lying physiological or constitutional change or variation. The varying organism has, like Hamlet, "that within which passeth show" and the superficial variations (like his "inky cloak" and other customary features of mourning) are but "the trappings and the suits" of a deep-lying change. Variation is not an inexplicable mystery, nor, on the other hand, are "varieties" sufficiently dealt with and their nature appreciated when one or two surface peculiarities are enumerated by which the collector can recognize them. A deeper study of the varying organism is both possible and needed.
If the gradual formation of new species from ancestral species is a true account of the matter, we must expect to find, at any rate here and there, if not frequently, traces of the process—for instance, gradations, or series of intermediate forms, connecting new, well-established species with the ancestral form or with one another. We do find such gradations—sometimes more, sometimes less, completely persisting over a wide tract of country, or discoverable in the fossiliferous deposits containing the remains of extinct animals.
For instance, when we look at the butterflies of a much larger region than our little island—namely, at those of a great continent like Africa or South America—we find that there are species which show gradations. Thus at a series of points, A, B, C, D, separated by some hundreds of miles from each other, we find a corresponding series of butterflies which are apparently closely similar species of one genus, differing by a few spots of colour, or darker and lighter tint, much as our Large White, Garden White, and Green-veined White differ. But when the butterflies are caught which occur at points intermediate between A and B, B and C, C and D, we find intermediate varieties, and, in fact, if we get a very large number from intermediate regions, we can, in some instances, arrange them in line so that they constitute a graduated series of forms, each being scarcely distinguishable from the one before or the one behind it, yet differing clearly from one a dozen places away. In such cases there is often evidence to show that the variety found at A breeds with that found at B, that of B with that of C, of C with D, so that they form an inter-breeding group, though perhaps the varieties at D will not pair with those at A, or even with those at B. Then sometimes we find in such a series, otherwise complete, a gap. Let us suppose it is between the butterflies of B and C. We find the series of gradations nearly complete, but some natural condition—such as the encroachment of the sea, or the slow elevation of a mountain range, or the climatic destruction of the necessary food-plant—has "wiped out" a few forms somewhere between those of B and C. They no longer exist. The series is no longer connected by inter-breeding forms; those occurring from A to B and some distance beyond are one "species" varying in the direction of the series C to D, but abruptly broken off from the latter. The series C to D is also a "species" with graduated varieties, but distinct; it is cut off from the lot once in continuity with it by the destruction of the intermediate forms inhabiting an intermediate area. Thus the one species becomes two, and these may again break up, and, having become thus disconnected and stabilized, they may spread over one another's territory—fly side by side and yet remain distinct forms which do not pair together—although originally they were varieties spreading from a common centre, where the ancestral species lived and multiplied.
Other similar gradational series of an interesting character have been noticed in the case of fresh-water fossil snail-shells. In the layers of clay and marl exposed by digging a railway cutting or a pit we may find that the successive layers represent a continuous deposit of 100,000 years or more, and we find sometimes that a form of snail-shell (not a species living to-day) occurs in the lowest stratum very different from that occurring in the highest stratum—the lowest being short and spherical, the highest elongated and of differing texture. In the intermediate layers, each 6 or 12 ins. thick and occupying perhaps altogether 30 ft. of vertical thickness, we find a graduated series of snail-shells leading almost imperceptibly from the oldest lowest form to the latest uppermost form. Such cases are known. But it is an exceptional thing to find these graduated series either spread over an area of the earth's surface, or following one another in successive strata. When they came into existence they were rapidly superseded and destroyed as a rule, and have left only one or two widely-separated examples of the intermediate forms. This we should naturally expect by analogy from what we know of the successive traces of human manufactures in the deposits on the site of some of the great cities of the ancient world which have been carefully excavated layer by layer. But still we have the important fact that here and there such gradational series have been found, and we are justified in considering a few isolated intermediate forms (which often occur connecting two greatly-differing species) as survivors of a former complete graduated series of intermediate forms, which came into existence by slow modification of an ancestral stock, and may, when the stock was widely spread over a continental area, not merely have succeeded one another in time, but actually coexisted in neighbouring regions.
There are many remarkable facts bearing upon the origin of "species," the description of which fills volumes written by such men as Darwin, Wallace, Poulton, and others, and become interesting to every one who has gained a correct notion of what naturalists mean by a "species." I will cite one in order to illustrate this. The bird which we call the red grouse, or nowadays simply "grouse" (the old Scotch name for it was "muir-fowl"), is one of twenty-four birds (among the 400 species of birds which live in the British Islands), including several kinds of titmouse, the goldfinch, bullfinch, song-thrush, stonechat, jay, dipper, and others which are very closely similar to species of birds living in Continental Europe, yet show some definite and constant marks, such as small differences in the colour of a group of feathers, enabling us to distinguish the British from the Continental forms. Are these twenty-four British forms to be regarded as distinct species?
The red grouse is placed in a genus called "Lagopus," of which there are several species in the northern hemisphere. In Scotland the red grouse, which is distinguished as Lagopus Scoticus, is accompanied by a rarer species of Lagopus, which lives in high, bare regions. This is the bird called by the Celtic name "ptarmigan"; it differs in several points from the red grouse, and acquires white plumage in the winter, which the latter bird does not; it is called Lagopus mutus. Now in Norway we find also two species of grouse or Lagopus, called "rypé" (pronounced "reeper") by the Norwegians. One is the same bird in every respect as the Scotch ptarmigan, and is known as "the mountain rypé." The other is very close to our red grouse, and is called "the common or bush rypé," and by English naturalists the "willow grouse," and by ornithologists "Lagopus salicetus." It agrees in habits, voice, eggs, and anatomical detail with our red grouse, but the back of the cock-bird of the red grouse and the whole plumage of the hen-bird have a darker colour. Moreover, the willow grouse, like the ptarmigan or mountain rypé, turns white—acquires a white plumage—in the winter which the red grouse does not. Are the red grouse and the willow grouse to be regarded as distinct species? Our British red grouse lives on heather-grown moors; the willow grouse prefers the shrubby growths of berry-bearing plants interspersed with willows, whence its name. Their food differs accordingly. Formerly the red grouse lived on the moors of the South of England, and when in Pleistocene times England was a part of the Continent of Europe the willow grouse and the red grouse were one undivided species inhabiting all the north-west of Europe. It is probable, though the experiment would be almost impossible to carry out, that were the eggs of a number of willow grouse now brought to Scotland and hatched on the moors, they would tend to keep apart from the native red grouse, and not inter-breed with them, in which case we should say that the Scotch form is a "species on the make," or, even, a completed and distinct species. On the other hand, it is possible that the two forms would freely pair with another, and that the colour and winter coat of the one (probably that of the Scotch form if the experiment were tried in Scotland) would predominate, and after some generations no trace of the other strain would be observable.
CHAPTER X
SOME SPECIFIC CHARACTERS
AN interesting case, showing that qualities which are life-preserving under certain severe conditions exist in some varieties of a species and not in others, was recorded some eight years ago. After a very severe "blizzard" 136 common sparrows were found benumbed on the ground by Professor Bumpus at Providence, United States. They were brought into a warm room and laid on the floor. After a short time seventy-two revived and sixty-four perished. They were compared to see if the survivors were distinguished by any measurable character from those which died. It was found that the survivors were smaller birds (the sexes and young birds being separately compared) than those which died, and were lighter in weight by one-twenty-fifth than the latter. Also, the birds which survived had a decidedly longer breastbone than those which died.
Similarly, the late Professor Weldon found that in the young of the common shore-crab, taken in certain parts of Plymouth harbour, those with a little peculiarity in the shape of the front of the shell survived when those without this peculiarity died. Many thousands were collected and measured in this experiment. It is not necessary to suppose that the distinguishing mark of the survivors in such cases is "the cause" of their survival. Such marks as the breadth of the front part of the crab's shell and the length of a bird's breastbone very probably are but "the outward and visible signs of an inward and (physiological) grace."
The marks, little peculiarities of colour and proportionate size, or some peculiar knob or horn, by which the student of species distinguishes one constant form from another, can rarely, if ever, be shown to have in themselves an active value in aiding or saving the life of the species of plant or animal. The mark or "character" is an accompaniment of a chemical, nutritional, physiological condition, and is in itself of no account. It is what is called "a correlated character." Such, for instance, is the black colour of the skin of pigs which in Virginia, U.S., are found, as stated by Darwin, not to be poisoned by a marsh plant ("the paint-root," Lachnanthes tinctoria), whilst all other coloured and colourless pigs are. The pigs which are not black develop a disease of their hoofs which rot and fall off, causing their death when they eat this special plant "the paint-root." The colour does not save the pig—it cannot correctly be called the cause of the pig's survival—but is an accompaniment of the physiological quality which enables the pig to resist the poisonous herb. So, too, with white-spotted animals. They are known to breeders as being liable to diseases from which others are free. Fantail pigeons have extra vertebræ in their tails, and pouter pigeons have their vertebræ increased in number and size. But the vertebræ were never thought of and "selected" by the breeders. They only wanted a fanlike set of tail feathers in the one case, and a longer body in the other. Some varieties of feathering maintained by pigeon breeders lead to the growth of abundant feathers on the legs (as in Cochin-China fowls), and it is found that these feather-legged pigeons always have the two outer toes connected by a web of skin. If it were a stabilized wild form we should separate it as a species on account of its webbed toes, yet the real selection and survival in the hands of the breeder had nothing to do with the toes or their web, but was simply "caused" by these pigeons having feathers of "survival or selection value" in his judgment. Male white cats with blue eyes are deaf. If deafness were ever an advantage (a difficult thing to imagine), you would get a species of cat with white hair and blue eyes, and be led to distinguish the species by those characters, not by the real cause of survival, namely, deafness. Not enough is yet known of this curious and very important subject of correlation, but its bearing on the significance of "specific characters" is sufficiently indicated by what I have said.
An interesting group of species, three of which are to be purchased alive through London fishmongers, are the European crayfishes, not to be confused with the rock-lobster or Langouste (Palinurus), sometimes called "crawfish" in London, nor with the Dublin prawn (Nephrops). The little river crayfishes are like small lobsters, and were placed by older naturalists in one genus with the lobsters. Now we keep the European species of crayfishes as the genus Astacus, and the common lobster and the American lobster have been put (by H. Milne-Edwards) into a separate genus (Homarus). You can buy in London the "écrevisses à pattes rouges" of French and German rivers, which is called Astacus fluviatilis, and differs from that of the Thames and other English and European rivers (which you can also buy) called A. pallipes ("pattes blanches" of the French), by the bright orange-red tips of its legs, and by having the side teeth of the horn or beak at the front of the head larger and more distinct. The English crayfish grows to be nearly as large as the "pattes rouges" in the Avon at Salisbury, though it has nearly disappeared about Oxford. You can also sometimes buy in London the big, long-clawed Astacus leptodactylus of East Europe. There are two or three other species, named and distinguished, which do not come into the London market.
Crayfishes, lobsters and the like have groups of plume-like gills (corresponding in the most ancient forms to the number of the legs and jaw-legs) overhung and hidden by the sides of the great shield or "head" of the animal. The common lobsters and crayfishes retain most of these in full size and activity, but have lost in the course of geologic ages the original complete number. These plume-like gills—each half an inch or so in length—are attached, some to the bases of the legs and some to the sides of the body above the legs. In the ancestral form there were thirty-two plumes on each side, twenty-four attached to the bases of the legs, and eight placed each at some distance above the connection of one of the eight legs with the side of the body. It is those on the side of the body which have suffered most diminution in the course of the development of modern crayfishes (and lobsters) from the ancestral form provided with the full equipment of thirty-two gill-plumes on each side. In fact, only one well-grown gill-plume, out of the eight which should exist on each side of the body-wall, is to be found—and that is the one placed above the insertion of the hindermost or eighth of the eight legs (eight when we reckon the three jaw-legs as "legs" as well as the five walking-legs). In front of this the side or wall of the body is bare of gill-plumes though they are present in full size on the basal part of most of the legs. Nevertheless, when one examines carefully with a lens the bare side of the body overhung by the head-shield or "carapace," one finds in a specimen of the common English "pale-footed crayfish" a very minute gill-plume high above the articulation of the seventh leg and another above the articulation of the sixth leg. They are small dwindled things, as though on the way to extinction, and are the mere vestiges of what were once well-grown gill-plumes, and still are so in the rock lobster and some prawns. In the red-footed crayfish of the Continent (Astacus fluviatilis) yet another minute vestige of a gill-plume is found, farther in front, on the body-wall above the fifth leg on each side of the animal. This furnishes a definite mark or character by which we can distinguish the red-footed crayfish from the common English pale-footed one. But these three rudimentary gill-plumes in the red-foot species, and two in the pale-foot species are all that until lately were recorded. The region of the body-wall above the fourth, third, second, and first of the legs was declared to be devoid even of a vestige of the branchial plumes which were there in ancestral forms, and have been retained more or less in some exceptional prawn-like creatures allied to the crayfish.
Zoologists take a special interest in the crayfish because it is found to be a most convenient type for the purpose of teaching the principles of zoology to young students, and with that end in view was made the subject of a very beautiful little book by the great teacher Huxley. The conclusions above stated in regard to the gills are set forth in that book with admirable illustrative drawings, and the striking fact of the dwindling and suppression of the various gill-plumes is clearly explained.
Fig. 33.—The rudimentary gill-plume of a crayfish from that part of the body-wall to which the first pair of jaw-legs (maxillipedes) is articulated. Found in the red-footed crayfish (Astacus fluviatilis) but in no other species of Astacus. It is one-fifteenth of an inch long. Drawn by Miss Margery Moseley in 1904. ("Quart. Journal of Microscopical Science," vol. 26 (1904-5).)
And now we come to an interesting discovery in this matter of the gill-plumes of crayfishes. Some fifteen years ago the daughter of my friend and colleague—Professor Moseley—was a member of the class of Elementary Biology at Oxford. She had to examine and identify these and other points in the structure of the crayfish. The class was supplied with specimens of the French red-footed crayfish "Astacus fluviatilis," as it is more readily obtained from fishmongers than our own "pale-foot" or "Astacus pallipes." She found in her specimen far forward on each side of the "head" a very minute gill far away from the others and previously unknown. The demonstrator in charge of the class refused even to look at her discovery. So she confirmed it by examining three other specimens—made drawings of the tiny branched gill (as shown in Fig. 33) and their position, and sent them to me in London. It was at once clear that she had discovered in this much studied little animal a very interesting pair of gills (right and left)—unknown to Huxley and the rest of the zoological world. She proceeded to examine specimens of A. fluviatilis from various rivers of Germany and France and always found the new gill-plume. She also showed (I supplied her with specimens at the Natural History Museum) that it was, on the other hand, absent from A. leptodactylus, A. pallipes, and all the foreign species (some from Asia) which are known, and she published an illustrated account of it in the "Quarterly Journal of Microscopical Science." This tiny gill-plume is placed very far forward on each side of the body, the farthest point forward at which any gill-plume is found in any kind of prawn, shrimp or lobster, namely in the region where the first pair of jaw-legs is attached, so that there are three empty spaces between it and the rudimentary gill over the fifth pair of legs, already known in the red-footed crayfish. It is only two millimetres long—about one-fifteenth of an inch! But its presence serves very distinctly to separate the red-footed crayfish, Astacus fluviatilis of French and German rivers, thus discovered to have four pairs of rudimentary gill-plumes, from the Astacus leptodactylus of the Danube basin and East Europe, which has only three pairs, and still more to emphasize the difference between it and our British species, the "white-foot" or Astacus pallipes, which has only two!
This little history is noteworthy, firstly, because it shows that a young student may, to use an appropriate term, "wipe the eye" of an expert observer and rightly venerated teacher (who would have delighted in the little discovery had he been alive), as well as the eyes of tens of thousands of students and teachers (including myself) who have studied the red-foot crayfish year after year, and missed the little gill. It is also interesting as showing us a good sample of a specific mark or character which has no survival value; that is, could not advantage the crayfish in the struggle for life. The fact is, that this one particular very minute forward pair of gill-plumes is like the other rudimentary gills—a survival in a reduced condition of a pair of gill-plumes which were well-grown, useful plumes aerating the blood, in the prawn-like ancestors of all crayfishes, lobsters, shrimps, and prawns, and is, owing to circumstances of nutrition and growth which we know nothing about but can vaguely imagine, retained by the red-foot species of crayfish, but lost by all other crayfishes, lobsters, common prawns and shrimps, and, in fact, only retained besides by a very few out-of-the-way kinds of marine prawns. That is the sort of thing which frequently has to serve as "a specific character" or mark, distinguishing one "species" from another.
A more ample discussion of the origin of species is not within the scope of this book. But I may say that until recently the conception that every organ, part and feature of a plant and animal must be explained, and can only be explained, as being of life-saving value to its possessor, and accordingly "selected" and preserved in the struggle for existence, was held by many "Darwinians" in too uncompromising a spirit. This conception was, really from the first, qualified by the admission that the life-saving value and consequent preservation of a structure must undoubtedly in some cases have been in operation in ancestors of the existing species, and is no longer operative in their descendants although they inherit the structure which has now become useless. Moreover, the operation of those subtle laws of nutrition and of form which are spoken of as the "correlation of parts in growth and in variation" (mentioned on p. 119) was pointed out by Darwin himself as probably accounting for many remarkable growths, structures and colour-marks which we cannot imagine to be now, or ever to have been in past ancestry, of a life-saving value. Nevertheless, the old "teleology," according to which, in pre-Darwinian days, it was held that every part and feature of an animal or plant has been specially created to fulfil a definite pre-ordained function or useful purpose, still influenced the minds of many naturalists. Natural selection and survival of the fittest were reconciled with the old teleological scheme, and it was held that we must as good Darwinians account for every structure and distinctive feature in every animal and plant as due to its life-saving value. Herbert Spencer's term, "the survival of the fittest," conduced to the diffusion of this extreme view: Darwin's equivalent term, "the preservation of favoured races," did not raise the question of greater or less fitness.
The extreme view is now, however, giving place to the recognition of the fact that the actual tendencies to variation—accumulated in the living substance of the various stocks or lines of descent and handed on during an immense succession of ages of change by hereditary transmission—counts for more in the production of new species and strange, divergent, even grotesque forms of both animals and plants than had been supposed.
Undoubtedly selection or survival of the fittest mainly accounts for the colouring and adaptive shaping of living things, and so for those several great types of modelling which arrest the eye and have excited the interest of inquisitive man. But there seems to be no justification for the assumption that in all cases a variation—that is to say, an increase or a diminution of the volume of some existing structure in proportion to other coexisting structures in the body of a living plant or animal—must be either favourable, that is, conducive to survival, or injurious, that is, tending to the defeat and destruction of its possessors or their race. On the contrary, it is the fact that there are vast areas and conditions related to countless myriads of living creatures in which variations of those creatures of large and imposing kind and degree are neither advantageous nor disadvantageous, but matters of absolute indifference, that is to say, without any effect upon the preservation or survival of their race or stock. Nature is far more tolerant than some of us were inclined to assume. In certain restricted conditions of competition and in regard to some special structures and components which are often so minute and obscure as to be not yet detected by that recent arrival, the investigating biologist—though sometimes, fortunately for him, large enough to jump to his eyes—it is undeniable that there must be a "survival" or "favouring" of individuals presenting a variation in increase, or it may be decreased, of this or that special feature of its "make-up" or structural components. But it is a more correct statement of the case to say that natural selection or survival preserves not the fittest, but the least fit possible under the circumstances—namely, all those which, however great their divagations and eccentricities of variation in other respects, yet at the same time attain to a minimum standard of qualification in those structures (or inner chemical qualities) essential for success in the competition for safety, food and mating determined by the particular conditions in which the competition is taking place. Consequently forms which are meaningless so far as standards of utility or "life-saving" are concerned, and are rightly described as grotesque, monstrous, gigantic or dwarfed—excessive (as compared with more familiar kinds) in hypertrophy or atrophy of their colouring and clothing, or of out-growths such as leaves of plants and limbs, jaws or other regions of the body of animals—are found existing in various degrees of eccentricity in every class of both plants and animals. Among animals such tolerated "exuberances of non-significant growth" are more striking than in plants. The group of fishes seems to be especially privileged in this way. They are freely variable in the position of the fins, the suppression or exaggeration of them, as well as of the scales on the surface of the body (e.g. leather carp and mirror carp). Take, for example, the mackerel and the salmon as standards of utilitarian adaptation of the body to an active life in sea or river, and then compare with theirs the astounding proportions of the sun-fish (Orthagoriscus) like a cherub "all head and no body," or the almost incredible Pteraclis—with its little body framed immovably between a huge dorsal and a huge ventral fin (see figures on p. 130). The fin-like crest of enormous size on the back of the great extinct lizard Dimetrodon of the Permian age supported by long bony spines is a similarly excessive and useless outgrowth. (This astonishing creature is shown in our Frontispiece.) Such exuberant products may be ascribed to an unrestrained "momentum" of growth which once set going by fortuitous variation has been tolerated but not favoured by natural selection. Or (as supposed by some) their excessive development may be due to the persistence of some nutritional condition which at first resulted in a moderate growth of the fin-like crests in question as a serviceable structure, but has persisted and increased long after the fin or crest has attained a sufficient size—simply because its increase though of no life-saving value—yet was not harmful and so did not bring its owner under the guillotine of natural selection. Such disproportionate exuberance of growth due to innate variability, tolerated but not specially favoured by natural selection, will account for many strange and grotesque forms of living things. From time to time in the long process of change, such exuberances may suddenly become of service and be, so to speak, taken in hand by natural selection, or they may become dangerous and lead to the extermination of the stock in which they have been previously tolerated.
Before my reader turns—as I hope he or she will do—to some handbook of zoology in which the genealogical tree or classification of the species of animals and of plants is treated at length, I will endeavour to give some estimate of the immense numbers of "species" which exist. As to mere individuals, it is impossible to form any estimate, but when we reckon up the teaming population of a meadow or forest in England, the hundreds of thousands of plants, including the smallest mosses and grasses, as well as the larger flowers, shrubs, and trees, the still greater number of insects, spiders, snails, and larger animals and birds, feeding on and hiding among them, and when we remember that in the ever-warm tropical regions of the earth life is ten or twenty times more exuberant than with us,—then the immensity of the living population of the land and water of the globe becomes as difficult to realize as are the figures in which the astronomer tells of the number and distances of the stars. On the other hand, some idea of the number of distinct species of animals and plants which have up to this date been recognized and described by naturalists as at present existing, may be formed by a statement of those which have been described in some of the more familiar groups. About 10,000 species of mammals have been described; about 14,000 of birds; 7000 of reptiles; 15,000 of fishes; 500,000 of six-legged insects; 14,000 of crustacea (shrimps, lobsters, crabs); 62,000 of molluscs (snails, mussels, etc.); 15,000 of star-fishes and sea-urchins; 5000 of corals and polyps; 3000 of sponges; and 6000 of microscopic protozoa. In all about 800,000 species of animals have been recorded, and probably as many more remain yet to be recognized and described.
The total number of described species of plants has never been estimated, but some idea of it may be formed from the fact that 1860 species of flowering plants alone have been distinguished in Britain, 17,000 in British India, and 22,000 in Brazil, not to mention those of Africa and Australia! These figures do not include the vast numbers of flowerless plants, the ferns, mosses, sea-weeds, mushrooms, moulds, lichens, and microscopic plants.
And then we have to add to these enumerations of living species the extinct species of successive geological ages, the remains of which are sufficiently well preserved to admit of identification. Those which are known are only a few thousands in number, and a mere fragment of the vast series of species which have existed in successive past ages of the earth. They are a few samples of the predecessors of the existing species, and some of them were the actual ancestors of those existing to-day. The larger number of them have left no direct issue, but represent side branches of the "tree of life" which have died out ages ago.
Strangely-shaped Fishes.—1. The Coffer-fish (Ostracion); 2. Pteraclis, an oceanic fish allied to the so-called Dolphins; 3. The Sun-fish (Orthagoriscus); 4. An Australian Blenny Patæcus.
CHAPTER XI
HYBRIDS
THE subject treated in this and the next chapter is one of the most interesting to mankind, and is surrounded by extraordinary prejudice, sentiment, and ignorance. It is one upon which really trustworthy information is to a very large extent absent—and difficult to obtain. I cannot profess to supply this deficiency, but I can put the matter before the reader.
It is a well-established fact that the various "kinds" of animals and of plants do not breed promiscuously with one another. The individuals of a "species" only breed with other individuals of that "species." They do not even, as a habit, breed with the individuals of an allied species. So nearly universal is this rule that it was for a long time held by naturalists to be an absolute definition of "a species," that it is a group of individuals capable of producing fertile young by breeding with one another and incapable of producing fertile young by mating with individuals of another such group, which were, therefore, held to constitute a distinct species. The practical importance of this definition was that it could, in a large number of instances among animals, and still more amongst plants, be made use of as a test and decided by experiment.
It is a curious fact that popular belief amongst country-folk and those who have opportunities of coming to a conclusion on so simple and direct a question has never accepted this law of the limitation of species in breeding as more than a general rule to which it has always been supposed that frequent exceptions occur. I mention this not in order to add that "there is always some basis of truth in these popular beliefs," but on the contrary to point out that popular beliefs on such matters are very frequently altogether erroneous, and though their origin can sometimes be explained, it is rare to find that they are due, in however small a degree, to true observation and inference. Where the subject under consideration has the obscurity and strong fascination for the natural man which all that relates to the processes of life, growth, and reproduction possess, we find that traditional fancies of the most unwarrantable kind are current, and hold their ground with tenacity even at the present day. Some 250 years ago, and earlier—in fact, before the commencement of that definite epoch of "the New Philosophy" marked by the foundation of the Royal Society of London—any queer-looking animal brought from remote lands, and any misshapen monstrosity born of cattle, sheep, dogs, or men, was "explained," and confidently regarded as a "hybrid," the result of a "cross" or irregular coupling of two distinct species of animals to which the "monster" presented some fanciful resemblance. Whole books were devoted to the description and picturing of such supposed examples of mis-begotten progeny.
The belief in the existence of such extraordinary hybrids is still common among so-called "well-educated" people. I have with difficulty avoided causing annoyance and offence to a friend, a celebrated painter, by refusing to admit that a deformed cat, of which he gave me an account, was a hybrid between a cat and a rabbit. A very eminent person whom I was conducting some years ago round the galleries of the Natural History Museum, declared, as we stood in front of the specimen of the Okapi of the Congo Forest, that it was clearly a hybrid between the giraffe and the zebra. He insisted that it was obvious that such was its explanation, and pointed to its striped haunches and legs, and its cloven hoofs and giraffe-like head. I failed to change his opinion.
It is the fact—ascertained by careful observation of natural occurrences and by experiment—that, in spite of the almost absolute law or general truth to the effect that the members of a species (whether of plant or animal) only produce fertile offspring by mating with members of that same species, yet there are rare instances known in which individuals of two distinct but allied species have mated and produced fertile offspring. The cases in which such unions have resulted in the production of offspring, but in which the offspring so produced prove to be infertile—that is, incapable of producing offspring in their turn—are much more numerous. An important distinction has also to be made between cases of either fertile or infertile hybrid-production which occur spontaneously in nature, and those in which man by separating the parent animals or plants from their natural conditions of life, or by bringing about impregnation (as in "pollinating" one flower with the pollen-dust of another) succeeds in obtaining a "cross" or "hybrid," whether fertile or infertile, not known to occur in "wild" (that is, not humanly controlled) nature. The rarest case would be that of the production of fertile hybrids in uncontrolled natural conditions. Such possibly occur in the case of some fishes in which the fertilization of the eggs takes place in water, the fertilizing microscopic sperms passing from the males like dust into the water and thus reaching the eggs laid by the females. Occasionally hybrids are thus produced between some common fresh-water fishes—species of the same genus—and between species of flat-fish, such as the turbot and the brill, though it is difficult to be sure that the rare hybrids so produced are fertile even if they attain to maturity. The same is true as to certain small flowering plants having distinct regions of natural distribution and occurrence. At the confines of the regions proper to two such allied species, insects passing from one to the other do sometimes effect a reciprocal fertilization of the two species, and a natural hybrid is the result. Here, again, it is difficult to follow the subsequent history of the hybrids, but it is believed that in some instances they are fertile, and that the hybrid race is only gradually merged by subsequent crossing into one or other of the parent species. Not a single instance is on record of the production of a "natural" hybrid (that is to say, one produced in natural conditions without man's interference), whether fertile or infertile, between two species of the larger animals (such as between horse and ass or zebra and ass, or between lion and tiger or any of the species of cats, or between species of bears) or birds (such as pheasants of various species, including the jungle cock, the wild original of our domestic fowl, or between various species of ducks, various species of geese, or between various species of the grouse-birds).
Nevertheless, in conditions brought about by man—that is to say, confinement in cages or paddocks, or at any rate removal from their native climate and home—all the groups of species just cited commonly and frequently produce hybrids inter se, that is, one or more species of the horse group thus inter-breed with one another, so will certain species of cats, certain species of bears, many species of pheasants, also of ducks, of geese, and of grouse. In nearly every case the hybrids so produced are infertile; they will not mate with a similar hybrid, and even when mated with one of the parent species rarely produce offspring, though they sometimes do so. The best cases of the production of fertile hybrids are between species of flowering plants brought to this country from widely separated regions. The surprising and instructive result has been obtained that a cross between two allied species (that is, of one and the same "genus") which will fail altogether or "come to nothing" as infertile hybrids—if the two species crossed are from the same or contiguous regions—yet will yield readily vigorous fertile hybrid offspring when the two species (always, of course, of one and the same genus) have their native homes in widely separate parts of the world—as, for instance, the Indian Himalaya range and the South American Andean range.
This has been found in crossing species of rhododendrons, of orchids, and of many other plants with which horticulturists occupy themselves for commercial purposes. It is in some ways the reverse of what one might expect. It would be reasonable to suppose that allied species from the same climate and geographical region would have more affinity and be more readily hybridized than species from widely remote and physically differing regions. But the reverse is the case, many thriving hybrid stocks which duly fertilize and set their seed are now in cultivation, having been produced by the union of parent species from "the opposite ends of the earth."
The consideration of this case throws some light on the significance of the non-occurrence of natural hybrids and of the very remarkable and curious fact that hybrids are so usually sterile. When we come to think of it, the natural preliminary assumption should be (as is that of unsophisticated humanity) that any animal or plant might, so far as possibilities go, breed with any other; and the questions to be answered are: (1) What advantage to a species is it not to be able to hybridize with other species, and (2) how—that is to say, by what structure or by what subtle chemical differences or other features in their make-up and habit—are they prevented from so hybridizing? Then we come on further to the question, Why should a hybrid, once produced, fail to bear healthy eggs or sperms according to its sex, although it grows up to full size and is to all appearances mature? And why should hybrids between parents of origin locally remote from one another not show this failure, but behave like ordinary healthy organisms?
In the full solution of these inquiries we should get very near to some of the most important secrets of the living body which have still to be searched out. But a reply to these questions which is probably in large measure true, and serves to help us in the further collection and examination of facts, is as follows: First, the production and maintenance of "species" of plants and of animals by survival of favourable variations in the struggle for existence (Darwin and Wallace's theory of the origin of species) requires the maintenance of the purity of the favourable stock which survives in the struggle. If it were continually liable to hybridization by other species it would never establish its own distinctive features. It would deteriorate by departing from those characteristics which have been "naturally selected" and have rendered it a successful "species." Thus the breeder, when he has selected a stock for propagation which approaches the standard at which he is aiming, keeps it apart, and does not allow it to be "crossed" by other stock. One of the qualities "naturally selected" in "the wild" is the power of resistance to fertilization by neighbouring species.
This power of resistance or immunity to fertilization by other species may be attained by several different methods. Amongst these are (1) a difference in the season of breeding or sexual ripening; (2) the production of secretions (whether by plant or by animal) which poison or paralyse the fertilizing sperms of allied and locally associated species, but are harmless to those of the secreting species; (3) the mechanical differences of size, etc., which prevent the fertilizing material of a strange species from gaining access to the egg-cells; (4) psychical activities (antipathies) in the case of animals or mere attraction and repulsion by odoriferous substances, which serve to repel a strange species, but are attractive to individuals of the same species; (5) finally, a chemical and physiological incompatibility between the sperms of one species and the germs of another (as distinct from the attraction or repulsion of the entire living individual), which, even when all other difficulties are absent or have been overcome, may be, and frequently is, present, so that the spermatozoon cannot penetrate the egg-cell even when resting upon it, but may be paralysed or repelled, and in any case is not guided and drawn into the aperture of the egg-covering, called the micropyle, or "little entry," so as to fuse with and fertilize the egg.
The operation of these hindrances to hybrid fertilization and breeding have been ascertained in several different instances. It is not always possible, and certainly not easy to ascertain, which is at work in any and every case. But we can well conceive that one or other of these agencies have been developed and accentuated by survival of the fittest, so as to protect a species against fertilization by a neighbouring species, and thus to enable it to maintain its own "bundle of characteristics" free from the swamping effects of "mixture" (that is, "hybridization") with another species. It is also thus intelligible that an allied species from a distant land against which our native species and its closer ancestry—struggling for purity of race—have had no occasion or opportunity to develop a repelling protection—will have no such difficulty in effecting the fertilization of the native species as have those adjacent species against whose intrusions the latter is specifically moulded and selected by long generations of severe natural selection.
The failure of hybrids generally to ripen their ova and sperm so as to reproduce themselves is a subject upon which, considering its enormous importance and significance, singularly little has been done in the way of investigation. Fifty years ago it was usually taught that the mule, between the horse and the ass, so largely produced under human superintendence for transport service, was unable to breed owing to some deformity in the reproductive passages. Even now no adequate study of the subject has been made, but it appears that whilst a female mule can be, and sometimes is, successfully mated to a horse or an ass, giving birth to a foal, the male mule does not produce fully-formed spermatozoa. What precisely is the nature of this failure, what the ultimate microscopic condition of the sperm cells in infertile male mules, or in any other infertile male hybrids, has not yet been properly worked out by modern cytological methods. It would be a matter of vast interest to determine what is the difference in the structure of the sperm-cells of a fertile and of an infertile male hybrid. At present, so far as I know, this has not been done.
So far what I have written applies to hybridization—the inter-breeding of distinct species. A similar but by no means identical subject is that of the inter-breeding of distinct races or varieties of one species, and the production of "mongrels." "Mongrels" are to races what "hybrids" are to species. To this branch of the subject belongs the study of the effects of intermarriage between distinct races of men.
CHAPTER XII
THE CROSS-BREEDING OF RACES
WE have seen that there is no simple rule as to the "mating" of individuals of a species with individuals of another closely allied but distinct species. Such mating very rarely comes about in natural conditions, but man by his interference sometimes succeeds in procuring "hybrids" between allied species. Hybrids between species belonging to groups so different as to be distinguished by zoologists as distinct "families" or "orders" are quite unknown under any circumstances. Such remoteness of natural character and structure as is indicated by the two great divisions of hoofed mammals—the even-toed (including sheep, cattle, deer, antelopes, giraffes, pigs and camels), and the odd-toed (including tapirs, rhinoceroses, horses, asses and zebras) is an absolute bar to inter-breeding. So, too, the carnivora (cats, dogs, bears and seals, and smaller kinds) are so remote in their nature from the rabbits, hares and rats—called "the rodents"—that no mating between members of the one and the other of these groups has ever been observed, either in nature or under artificial conditions.
Even when individuals of closely allied species mate with one another it is a very rare occurrence that the hybrids so produced ripen their ova and sperms so as to be capable of carrying on the hybrid race, though sometimes they do ripen them and breed. The great naturalist Alfred Wallace, in his most valuable and readable book called "Darwinism," expressed the opinion that the apparent failure of hybrid races to perpetuate themselves by breeding was to a large extent due to the small number of individuals used in experiments on this matter, and the in-and-in breeding which was the consequence. One of the great generalizations established by Darwin is that in-and-in breeding is, as a rule, resisted in all animals and plants, and leads when it occurs to a dying-out of the inbred race by resulting feebleness and infertility. A large part of Darwin's work consisted in demonstrating the devices existing in the natural structure and qualities of plants and animals for securing cross-fertilization among individuals of the same species but of different stock. Both extremes seem to be barred in nature—namely, the inter-breeding of stocks so diverse in structure and quality as to be what we call "distinct species," and again the inter-breeding of individuals of the same immediate parentage or near cousinship. What seems to be favoured by the natural structure and qualities of the plant or the animal is that it shall only breed within a certain group—the species—and shall within that group avoid constant self-fertilization or fertilization by near cousins. Thus we find numerous cases in which, though the same flower has both pollen and ovules, and might fertilize itself, the visits of insects (specially made use of by mechanisms in the flower) carry the pollen of one flower to the ovules of another and to flowers on separate plants growing at a distance. It is necessary to note that there are, nevertheless, self-fertilizing flowers, and also self-fertilizing lower animals, the special conditions of which require and have received careful examination and consideration, upon which I cannot now enter.
In relation to the question of the possibility of establishing hybrids between various species experimentally, I must (before going on to the cognate question of "mongrels") tell of an interesting suggestion made to me by my friend Professor Alphonse Milne-Edwards not long before he died, and never published by him. He was director of the Jardin des Plantes in Paris, where there is a menagerie of living beasts as well as a botanic garden and great museum collections and laboratories. He held it to be probable, as many physiologists would agree, that the fertilization of the egg of one species by the sperm of another, even a remotely related one, is ultimately prevented by a chemical incompatibility—chemical in the sense that the highly complex molecular constitution of such bodies as the anti-toxins and serums with which physiologists are beginning to deal is "chemical"—and that all the other and secondary obstacles to fertilization can be overcome or evaded in the course of experiment. He proposed to inject one species by "serums" extracted from the other, in such a way as seemed most likely to bring the chemical state of their reproductive elements into harmony, that is to say, into a condition in which they should not be actively antagonistic but admit of fusion and union. He proposed, by the exchange of living or highly organized fluids (by means of injection or transfusion) between a male and female of separate species, to assimilate the chemical constitution of one to that of the other, and thus possibly so to affect their reproductive elements that the one could tolerate and fertilize the other. The suggestion is not unreasonable, but would require a long series of experiments in which the possibility of producing such "assimilation," even to a small extent and in respect of less complex processes than those ultimately aimed at, would have to be, first of all, established. My friend did not live to commence this investigation, but it is possible that some day we may see the obstacle to the union of ovum and sperm of species, which are to some extent allied, removed in this way by transfusion or injection of important fluids from the one into the other.
We must not lose sight of the fact, in the midst of these various and diverging observations about the fertilization of the ova of one species by sperms of another species, that there is such a thing as "parthenogenesis," or virgin-birth. In some of the insects and lower forms of animals the egg-cell habitually and regularly develops and gives rise to a new individual without being fertilized at all. And in other cases by special treatment, such as rubbing with a brush, or in the case of marine animals by addition of certain salts to the water in which the eggs are floating—or, again, in the case of the eggs of the common frog by gently scratching them with a needle—the eggs which usually and regularly require to be penetrated by and fused with a spermatozoon or sperm-filament before they will develop, proceed to develop into complete new individuals without the action upon them of any spermatozoon. In such marine animals as the sea-urchins or sea-eggs it has been found that the eggs deposited in pure sea-water, though they would die and decompose if left there alone, can be made to develop and proceed on their growth by the addition to the sea-water of the sperm filaments of a star-fish (the feather star or comatula). The spermatozoa or sperm-filaments do not, however, in this case fuse with the egg-cells. They mechanically pierce the egg-coat, but contribute no substance to the embryo into which the egg develops. They have merely served, like the scratch of a needle on the frog's egg and the brushing of insects' eggs, to start the egg on its growth, to "stimulate" it and set changes going. It appears thus that the fertilizing sperm-filaments of organisms generally have two separate and very important influences upon the egg-cells with which they fuse. The one is to stimulate the egg and start the changes of embryonic growth; the other is to contribute some living material from the male parent to the new individual arising from the growth and shaping of the egg-cell. The first influence can be exercised without the second, as is seen in the case of the eggs of some sea-urchins stimulated to growth by the spermatozoa of some star-fishes. It happens that these marine animals are convenient for study and experiment because their eggs are small and transparent and that they and the spermatozoa are freely passed into the sea-water at the breeding season, in which the fertilization of the eggs takes place.
When these facts are considered we have to admit that in the mating of two species which will not regularly and naturally breed together, there may be a limited action of the spermatic element which may stimulate the egg to development without contributing by fusion in the regular way to the actual substance of the young so produced, or only contributing an amount insufficient to produce a full and normal development of the hybrid young. Such cases not improbably sometimes occur in higher animals, though they have not been, as yet, shown to exist except in the experiments with sea-urchins' eggs and feather-star's sperm.
In all animals and plants, but especially in domesticated and cultivated stocks or strains, varieties arise which, by natural or artificial separation, breed apart, and give rise to what are called "races." Such races in natural conditions may become species. Species are races or groups of individuals, which, by long estrangement (not necessarily local isolation) from the parent stock and by adaptation to special conditions of life, have become more or less "stable"—that is, permanent and unchanging in the conditions to which they have become adapted. They acquire by one device or another the habit of not breeding with the stock from which they originally diverged—a repugnance which may be overcome by human contrivance or by natural accident, but is, nevertheless, an effective and real quality. Distinct forms, which have not arrived at the stability and separation characteristic of species, are spoken of as "races," or "varieties." It is very generally the case that the "races" of one species can inter-breed freely with one another, and with the original stock, when it still exists. Comparatively little is known as to the behaviour of wild or naturally-produced "races." Practically all our views on the subject of "races" and their inter-breeding are derived from our observation of the immense number and range of "races" and "breeds" produced by man—as farmer, fancier, and horticulturist. It has been generally received as a rule, that the various races produced in the farm or garden by breeding from a species, will inter-breed freely, and produce offspring which are fertile. A special and important series of races, in which human purpose and voluntary selection necessarily have a leading part, are the races of man.
The offspring of parents of two different races is called a mongrel, whilst the term "hybrid" has been of late limited, for the sake of convenience, to the offspring of parents of two different species. Mongrels, it has been generally held, are fertile—often more fertile than pure-bred individuals whose parents are both of the same race, whilst "hybrids" are contrasted with them, in being infertile. We have seen that infertility is not an absolute rule in the case of hybrids, and it appears that there is also a source of error in the observations which lead to the notion that "mongrels" are always fertile. The fact is that observations on this matter have nearly always been made with domesticated animals and plants which are, of course, selected and bred by man on account of their fertility, and thus are exceptionally characterized by fertility, which is transmitted in an exceptional degree to the races or varieties which are experimentally inter-bred, and, consequently, may be expected to produce fertile mongrels. Alfred Russel Wallace insisted upon this fact, and pointed out that in a few cases colour varieties of a given species of plant have been found to be incapable of inter-breeding, or only produce very few "mongrels." This has been established in the case of two dissimilarly-coloured varieties of mullein. Also the red and the blue pimpernel (the poor man's weather-glass, Anagallis), which are classed by botanists as two varieties of one species, have been found after repeated trials to be definitely incapable of inter-breeding. Wallace insists in regard to crossing, that some degree of difference favours fertility, but a little more tends to infertility. We must remember that the fertility of both plants and animals is very easily upset. Changed conditions of life—such as domestication—may lead (we do not know why) to complete or nearly complete infertility; and, again, "change of air," or of locality, has an extraordinary and not-as-yet-explained effect on fertility.
"Oh, the little more and how much it is!
And the little less, and what worlds away!"
Infertile horses sent from their native home to a different climate (as, for instance, from Scotland to Newmarket) become fertile. A judicious crossing of varieties or races threatened with infertility will often lead to increased vigour and fertility in the new generation, just as change of locality will produce such a result. Physiological processes which are not obvious and cannot be exactly estimated or measured are then, we must conclude, largely connected with the question of sterility and fertility. Mr. Darwin has collected facts which go far to prove that colour (as in the case of the black pigs of Virginia, which I cited in Chapter X.), instead of being a trifling and unimportant character, as was supposed by the older naturalists, is really one of great significance, often correlated with important constitutional differences. It is pointed out by Alfred Wallace that in all the recorded cases in which a decided infertility occurs between varieties (or races) of the same species of plants (such as those just cited), those varieties are distinguished by a difference of colour. He gives reasons for thinking that the correlation of colour with infertility which has been detected in several cases in plants may also extend to animals in a state of nature. The constant preference of animals—even mere varieties of dog, sheep, horses, and pigeons—for their like, has been well established by observation. Colour is one of the readiest appeals to the eye in guiding animals in such selection and association, and is connected with deep-seated constitutional qualities. "Birds of a feather flock together" is a popular statement confirmed by the careful observation of naturalists. Thus we arrive at some indication of features which may determine the inter-breeding, or the abstention from inter-breeding, of diverse races sprung from one original stock. The "colour bar" is not merely the invention of human prejudice, but already exists in wild plants and animals.
We now come to the questions, the assertions, the beliefs, and the acts concerning the inter-breeding of human races, to the consideration of which I have been preparing the way. The dog-fancier has generally a great contempt for "mongrels." Breeders generally dislike accidental crosses, because they interfere with the purpose which the breeder has in view of producing animals or plants of a quality, form, and character which he has determined on before-hand. This interference with his purpose seems to be the explanation of beliefs and statements, to the prejudice of "mongrels." Really, as is well known to great breeders and horticulturists, a determined and selective crossing of breeds is the very foundation of the breeder's art, and there is no reason to suppose that a "mongrel" is necessarily, or even probably, inferior in vigour or in qualities which are advantageous in the struggle for life in "natural"—that is to say, "larger"—conditions of an animal's or plant's life; not those limited conditions for which the breeder intends his products. Indeed, the very opposite is the case. In nature, as Mr. Darwin showed, there are innumerable contrivances to ensure the cross-breeding of allied but distinct strains. Dog-owners who are not exclusively bent upon possessing a dog which shows in a perfect way the "points" of a breed favoured by the fashion of the moment, or fitting it for some special employment, know very well that a "mongrel" may often exhibit finer qualities of intelligence, or endurance, than those exhibited by a dog of pure-bred "race." And the very "races" which are spoken of to-day as "pure-bred," or "thoroughbred," have (as is well known) been produced as "mongrels"—that is to say, by crossing or mating individuals of previously-existing distinct and pure breeds. The history of many such "mongrel breeds," now spoken of as "thoroughbred," is well known. The English racehorse was gradually produced by the "mongrelizing," or cross-breeding, of several breeds or races—the English warhorse, the Arab, the Barb. A very fine mongrel stock having at last been obtained, it was found, or, at any rate, was considered to be demonstrated, that no further improvement (for the purposes aimed at, namely, flat-racing) could be effected by introducing the blood of other stock. The offspring of the "mongrels" Herod, Matchem, and Eclipse accordingly became established as "the" English racehorse, and thenceforward was mated only within its own race or stock, and was kept pure or "thoroughbred." Another well-known mongrel breed which is now kept pure, or nearly so, is that of the St. Bernard's dog, a blend of Newfoundland, Bloodhound, and English Mastiff.
Often the word "mongrel" is limited in its use to signify an undesired or undesirable result of the cross-breeding of individuals of established races. But this is not quite fair to mongrels in general, since, as we have seen, the name really refers only to the fact they are crosses between two breeds. When they happen to suit some artificial and arbitrary requirement they are favoured, and made the starting-point of a new breed, and kept pure in their own line; but when they do not fit some capricious demand of the breeder they are sneered at and condemned, although they may be fine and capable animals. No doubt some mongrels between races differing greatly from one another, or having some peculiar mixture of incompatible qualities the exact nature of which we have not ascertained, are wanting in vigour, and cannot be readily established as a new breed. In nature the success of the mongrel depends on whether or not its mixture of qualities makes it fitter than others to the actual conditions of its life, and able to survive in the competition for food and place. In man's breeding operations with varieties of domesticated animals and "cultivated" plants, the survival of the mongrel depends upon its fitting some arbitrary standard applied by man, who destroys those which do not suit his fancy, and selects for survival and continued breeding those which do.
What is called "miscegenation," or the inter-breeding of human races, must be looked at from both these points of view. We require to know how far, if at all, the mixed or mongrel offspring of a human race A with a human race B is really inferior to either of the original stocks A and B, judged by general capacity and life-preserving qualities in the varied conditions of the great area of the habitable globe. And how far an arbitrary or fanciful standard is set up by human races, similar to that set up by the "fancier" or cultivator of breeds of domestic animals. The matter is complicated by the fact that what we loosely speak of as "races" of man are of very various degrees of consanguinity or nearness to one another in blood, that is, in stock or in ultimate ancestry. It is also complicated by the fact that we cannot place any reliance upon the antipathies or preferences shown by the general sentiment of a race in this (or other matters) as necessarily indicating what is beneficial for humanity in general or for the immediate future of any section of it. Nor have we any assurance that what is called "sexual selection"—the preference or taste in the matter of choosing a mate—is among human beings necessarily anything of greater importance—so far as the prosperity of a race or of humanity in general is concerned—than a mere caprice or a meaningless persistence of the human mind in favouring a choice which is habitual and traditional. I have referred to this point again in the last paragraph of this chapter.
In regard to marriage between individuals of different European nationalities, a certain amount of unwillingness exists on the part of both men and women which cannot be ascribed to any deep-seated inborn antipathy, but is due to a mistrust of the unknown "foreigner," which very readily disappears on acquaintance, or may arise from dislike of the laws and customs of a foreign people. English, French, Dutch, Scandinavians, Germans, Russians, Greeks, Italians and Spaniards have no deep-rooted prejudices on the subject, and readily intermarry when circumstances bring them into association. Though the Jews by their present traditional practice are opposed to marriage with those not of their faith, there is no effective aversion of a racial kind to such unions, and in early times they have been very frequent. During the "captivity" in Babylon and again after the "dispersal" by the Romans, the original Jewish race was practically swamped by mixture with cognate Oriental races who adopted the Jewish faith. So far from there being inborn prejudice against intermarriage of the peoples above cited, it is very generally admitted that such "miscegenation" leads frequently to the foundation of families of fine quality. The blend is successful, as may be seen in the number of prominent Englishmen who have Huguenot, German, Dutch, or Jewish blood in their veins.
But when we come to the intermarriage of members of the white race of Europe with members of either the negroid (black) race or of the yellow and red mongoloid race, a much greater and more deeply-rooted aversion is found, and this is extended even to members of the Caucasian race who, possibly by prehistoric mixture with negro-like races, are very dark-skinned, as is the case with the Aryan population in India and Polynesia. It is a very difficult matter; in fact, it seems to me not possible in our present knowledge of the facts, to decide whether there is a natural inborn or congenital disinclination to the marriage of the white race, especially of the Anglo-Saxon branch of it, with "coloured" people, or whether the whole attitude (as I am inclined to think) is one of "pride of race," an attitude which can be defended on the highest grounds, though it may lead to erroneous beliefs as to the immediate evil results of such unions, and to an unreasonable and cruel treatment both of the individuals so intermarrying and of their offspring. There is little or no evidence of objection to mixed unions on the part of the coloured people with whites, no evidence of physical dislike to the white man or white woman, but, on the contrary, ready acquiescence.
A curious aversion to marriages with whites on the part both of North American Indians and of negroes is, however, recorded from time to time in the official reports of the United States Government.
Two beliefs about such unions are more or less prevalent among white men in the regions where they not infrequently occur. Neither of these beliefs is supported by anything like conclusive evidence. The one is that such unions lead to the production of relatively infertile offspring; the mixed breed or stock is said to die out after a few (some seven or eight) generations. It is, however, the fact that the circumstances under which this occurs suggest that it is not due to a natural and necessary infertility. The other assertion is that the offspring of parents—one of white race and the other of black, yellow or brown—tend by some strange fatality to inherit the bad qualities of both races and the good qualities of neither. This is a case to which must be applied the saying, "Give a dog a bad name and hang him." The white man in North America, in India, and in New Zealand desires the increase and prosperity of his own race. Like the fancier set on the production of certain breeds of domesticated animals, he has no toleration for a "mongrel." In so far as it is true that miscegenation (marriage of white and coloured race) produces a stock which rapidly dies out—this is due to the adverse conditions, the opposition and hostility to which the mixed race is exposed by the attitude of the dominant white race. To the same cause is due the development of ignoble and possibly dangerous characteristics in the unfortunate offspring of these marriages more frequently than in those who find their natural place and healthy up-bringing either in the white or the coloured sections of the community. The "half-breed" is in some countries inexorably rejected by the race of his or her white parent and forced to take up an equivocal association with the coloured race.
That some, at any rate, of the evils attributed to "miscegenation" are due to the baneful influence of "pride of race" is evident from the fact that the Portuguese (with the exception of a small aristocratic class) have not since the early days of the fourteenth century, perhaps in consequence of established association with the Moorish and other North African races, shown that pride of race and aversion to mixture with dark-skinned races which is so strong a feature in the Anglo-Saxons, their successors and rivals as colonists. The long-standing admixture of black blood in the Portuguese population before the colonization of South America, has led to a toleration on the part of the Portuguese colonists of "miscegenation," both with Indians and the liberated descendants of imported negro slaves. The consequence is that in Brazil there is no condemnation of black blood; children of mixed parentage and of coloured race attend the same schools as those of European blood, and freely associate with them. There is no notion that that portion of the population which is of mixed negro, Indian, and white blood is less vigorous or fertile than the unmixed, nor that vice and feebleness are the characteristics of the former, whilst virtue and capacity belong to the latter.
The determined hostility of the Anglo-Saxon race in North America and in British India to "miscegenation" is in the case of the United States to be explained by the peculiar relation of a large slave population in the Southern States to a pure white slave-owning race: whilst in India we have a handful of white men temporarily stationed as rulers of millions of "natives," but never accepting India as their home. The attitude of the Anglo-Saxon race to the North American Indians, and also to the Maoris of New Zealand, has never been so extreme in the matter of miscegenation as it has been to negroid people and to the very different though dark-skinned people of the East. In support of that opinion may be cited the fact that some of "the first families of Virginia" are proud of their descent from Pocahontes, the Algonkian "Princess" who married the Englishman Rolfe. In New Zealand there are many families of mixed Anglo-Saxon and Maori blood. Though they are not ostracized, as are the half-breeds of negro blood in the United States, there is a firm tendency to relegate the half-breeds in New Zealand to the Maori section of the population, which it must be remembered includes some of the richest and most prosperous landowners in the colony.
It may be questioned whether there is in this matter a greater "pride of race" among Anglo-Saxons than among other Northern European peoples. Neither the French nor the Germans have established great colonies like the English, nor undertaken the administration of a huge Eastern Empire, and have, therefore, not shown what attitude they would adopt under such circumstances. The tolerance and easy-going humanitarianism of the French in relation to "miscegenation" in their dependencies in past times has never had the significance or practical importance which it would have possessed in the English Colonies and in the great Indian Empire.
There is, on account of the sporadic and exceptional occurrence of modern instances, no information of any value as to the results of mixture of other races of man. In early times and among more primitive or less civilized peoples there appears to have been, when immigration or conquest gave the opportunity, no obstacle to a free intermixture of an incoming race with the natives of an invaded territory. The "pride of race" has, nevertheless, throughout historic time been a frequent factor in the adjustment of populations of diverse races, and though "colour" has been a frequent "test" or symbol of the superior and exclusive race, it has not been the only characteristic exalted to such importance. Such "pride of race" has frequently excluded the members of a closely allied but conquered racial group from intermarriage with the conquerors, and has only disappeared after centuries of persistence. The term "blue blood" is interesting in this connection. It is the "saing d'azure" of the Gothic invaders, the conquerors of the Iberian and Moorish people of Spain. It refers not to any "blueness" of the blood itself, such as distinguishes veinous from arterial blood, but to the blue colour of the veins as seen through the colourless skin of a northern race (the Goths), as compared with the invisibility of the veins when the skin is rendered more or less opaque by a brown pigment, as in the Moors and the swarthy Iberians.
Among the people of Western Europe marriage has assumed more and more a character which is almost unknown in the rest of the world. Whatever the future may be in regard to this matter, there is no doubt possible that the place given to women in Western Europe by the ideals of chivalry and the practice of the northern race (which has so largely displaced the traditions of the Roman Empire) has established a relation of the sexes in which marriage and consequent parentage have ceased to be regarded as a mere regularization of animal desire and appetite. The accepted, but not always consciously recognized, view of marriage in Western Europe is that the union so sanctioned and the families thereby produced should be the result not of the mere physical necessity of irresponsible victims of an impulse common to all animals, but the outcome of the deliberate choice of man and woman attracted to one another by sympathy, understanding and reciprocal admiration, based upon knowledge of character, mental gifts and aspirations, as well as upon bodily charm. A rarely-expressed but none the less deeply-seated conviction exists that from such unions children of the finest nature, nurtured in circumstances most likely to make them worthy members of the community, will be born and reared. It is this conviction which leads to, or at any rate endorses, the exclusiveness which is described as "pride of race." The Anglo-Saxon man and equally the Anglo-Saxon woman (as well as the allied races of neighbouring nationalities) recognize a responsibility, a race duty, resulting from accumulated tradition, the heirloom of long ages of family life, which causes the man to be ashamed of, and the woman to shrink with instinctive horror from, union with an individual of a remote race with whom there can be no real sympathy, no intimate understanding. That seems to me to be the explanation and the justification of the "colour bar."
In relation to the probable effectiveness of sexual selection among uncivilized peoples in favouring and maintaining a particular type or form of features, hair, etc., characteristic of the race, independently of the life-preserving value of such qualities, I may mention, before quitting this difficult but strangely fascinating subject, a fact observed by a traveller in Africa, and related to me by him. Other similar facts are on record. Among the negroes employed as "porters" by my friend, some thirty in number, was one who had a narrow aquiline nose and thin lips. He was as black and as woolly-haired as any of them, but would if of fair complexion have been regarded by Europeans as a very handsome, fine-featured man. Such cases are not uncommon in parts of Africa, where probably an unrecognized mixture with Arab or Hamite blood has occurred. My friend expected this man to be a favourite, on account of what to him appeared to be "good looks," with the girls of the villages at which he camped during a three months' journey. At every such village, as they journeyed on, the travellers were received with joy and good nature. The negro porters were fêted and made much of by the young women. But one alone was unpopular and regarded with ridicule and dislike. This was the handsome negro with the fine, well-modelled nose and beautiful European lips. The black beauties turned their backs on him, in spite of his amiable character and kindly overtures. They invariably and by open confession preferred the men with the thickest lips, the broadest noses, and the most thoroughly (as we should say) degraded prognathous appearance and disgusting expression. Hence no doubt the young negresses were likely to perpetuate in their offspring the features which are characteristic of their race, and hence it is probable that mere capricious sexual selection of individuals most completely conforming to a preferred type—irrespective of the value of the features preferred—may have great effect in both the selection and the maintenance of the peculiarities of the type. Dark skin may thus have been selected, until it became actually black; a slight curling of the hair, until it became woolly; thickish lips and broadish nose, until they became excessive in thickness and breadth.
CHAPTER XIII
WHEEL ANIMALCULES
TWO hundred years ago the Dutch naturalist Leuwenhoek, who made many discoveries with the highly magnifying lenses which he himself ground and mounted, wrote to the Royal Society of London that he had "discovered several animalcula that protrude two wheels out of the forepart of their body as they swim, or go on the sides of the glass jar in which they are living." He says that "the two wheels are thick set with teeth as the wheel of a watch," and he sent to the society for publication drawings of these wonderful little creatures. This was the first account of the Wheel Animalcules. Since then they have been studied by many microscopists, especially by Ehrenberg, who figured many in his great book on animalcules in 1838. Fourteen years later the delightful English naturalist, P. H. Gosse, who studied and illustrated the "sea-anemones" so ably—and, by his example and charming descriptions, made the keeping of these beautiful things in marine aquaria a favourite occupation among people of leisure, blessed with a "curiosity concerning the things of nature"—published some microscopical studies on Wheel Animalcules, and continued throughout his life to make them a special subject of his investigation.
The microscope was greatly improved—in fact, reached its present state of perfection—during Mr. Gosse's lifetime, and a wonderful amount was added to our knowledge not only as to the various kinds of wheel animalcules (which now number not less than 900 species), but also with regard to the minutest details of their structure, their growth from the egg, and their habits. Another English lover of these minute creatures, Dr. C. T. Hudson, of Clifton (Bristol), began his observations a few years later, and also discovered many wonderful kinds. It was my good fortune to bring these two devotees of the Rotifera, or Wheel Animalcules, together, and to induce them to write a conjoint work on their favourites—after, as they say in their preface, they had each continued their studies almost daily for thirty years, and had made innumerable drawings from living specimens, which are reproduced in the many hundred (mostly coloured) figures engraved in the thirty-four quarto plates of their monumental book. This was published in 1889, a year after Mr. Gosse's death at the age of 78. My friend, Mr. Edmund Gosse, the distinguished man of letters, is the son of the naturalist; the microscope, the aquarium, and the rock-pools of the seashore were the familiar delights of his boyhood, as of mine.
In Fig. 34 I have sketched the common Rotifer or wheel animalcule. It is about the one-fortieth of an inch long. The two specimens drawn in Figs. 34, A and B, are seen to be clinging by the forked tail-end of the body to a piece of weed (drawn in dotted lines). The body is stretched in these specimens to its full length. It can be shortened by a "telescoping" or pulling in of either end, so as to make the animal a mere oval particle. The four narrower joints or segments at the tail-end can be pulled in like the segments of a telescope, whilst the two wheels and adjacent parts can be drawn down into the body as shown in Fig. 34, C, where the two wheels (W) are seen showing through the skin by transparency.
Fig. 34.—Diagram of Rotifer vulgaris—the common wheel animalcule—one hundred and twenty times as long as the creature itself. A, front view. B, side view. C, head showing eyes S, and retracted wheel apparatus W. The letters in A and B have the following signification: M, mouth. W, wheel or ciliated disc. S, eye spots on head. T, spur or tentacle. G, gizzard. St, stomach. Int, intestines. V, vent: aperture of intestine.
The common rotifer can walk like a looping caterpillar or a leech—fixing itself by its tail, then stretching out the head and fixing that, whilst letting go the tail and bringing it up by "telescoping" it, near to the head region. The tail is forked, and in the side view (Fig. 34, B) it is seen to have a soft branched process, which helps it to cling. The letter V in Fig. 34, A, points to the vent or opening of the gut at the fork of the tail. The mouth, marked M, is seen between the two "wheels." The two "wheels" are really two discs, the edges of which are beset by coarse "cilia," or vibrating hairs of protoplasm. [5] These cilia "lash" and straighten again one after the other, so that the optical illusion is produced of the toothed edge of the disc being in movement like a wheel. They may be "focused" with the microscope so that the groups or "bunches" of them look like stiff, motionless "teeth," although they are really, all the time, lashing and beating in regular rhythm. When the animal is fixed by its tail, the lashing of the cilia on the wheels causes currents in the water which set with great strength to the mouth and bring floating food particles to it. It is thus that the Rotifer feeds. When the tail is not grasping a support, the movement of the cilia on the wheels causes the animal to swim forward through the water, so that it has two modes of locomotion—the leech-like crawling method and the free swimming method.
The various internal organs of a Rotifer are readily seen through its transparent skin (Fig. 34, A). It has a nervous system, many bands of contractile muscles and a pair of little tubular kidneys or nephridia, besides reproductive germs (the eggs). I have here sketched only the digestive canal. The mouth leads through a gullet to a very curious organ called the "gizzard," marked G. All the wheel animalcules have this gizzard, but its teeth, shown as two oval bodies in the drawing, differ a great deal in shape and complexity in the different kinds. Whilst the Rotifer is feeding by bringing currents of water to its mouth, the two halves of the gizzard are kept in rapid movement by muscles, causing them to rub against one another and to grind up the food particles which reach them through the gullet. The gizzard (G) is followed by the digestive stomach (St), and that by the intestine (Int), which opens at the vent (V). The side (or three-quarter profile) view of a similar specimen (Fig. 34, B) shows only the surface of the little animal, and is intended to show especially the snout-like head-lobe (S), with its two eye-spots, which are red in colour. Standing out backwards from this is a finger-like process (T), which is called the spur, or tentacle. It has hairs at its tip, and is a sensory organ.
Fig. 35.—The Rotifer Pedalion mirum—seen from the right side, magnified 180 diameters. w.a., wheel apparatus or "ciliated" margin of the cephalic disc. r.e., right side eye-spot. m., mouth. p., tactile process. d.l., median dorsal limb (as it is seen in profile, only three of the fringed hairs at its extremity are seen). v.l., the great ventral limb (only five of its fan of eight fringed hairs are seen). l.l.1, dorso-lateral, and l.l.2, ventro-lateral limbs of the right side: they show the complete fans of eight fringed hairs. x., the pair of posterior processes tipped with vibratile cilia, better seen in Fig. 36.
Fig. 36.—The Rotifer Pedalion mirum—seen from the ventral surface. Letters as in Fig. 35. The complete fan of eight fringed hairs terminating the great ventral limb are seen, and the three spine-like processes on each side of it. The fringed hairs of the two ventro-lateral limbs, l.l.2, are omitted; they are fully shown in Fig. 35, and are the same in number and disposition as those forming the "fan" of the great ventral limb. Compare these hairs with those of the "Nauplius" Crustacean larva drawn as a tail-piece to Chapter XIII.
In some wheel animalcules there is a pair of these spurs, and the very remarkable wheel animalcule drawn in Figs. 35 and 36 has six large processes which, though much bigger, appear to be of the same nature. Of these four are seen in Fig. 35, namely, d.l., the dorsal limb, v.l., the great ventral limb, and l.l.1 and l.l.2, the two lateral limbs of the right side, all of them carrying fan-like groups of fringed hairs. They are moved by very powerful muscles, and strike the water with energetic strokes, so as to cause the little owner to dart through it. This jumping or darting wheel animalcule is called "Pedalion," and was discovered and described by Dr. Hudson. It is so astonishing and wonderful a little beast, that when Dr. Hudson sent me some alive in a tube by post in 1872, soon after he had discovered it, I could not believe my eyes, and thought I must be dreaming. It is very like the young form of Crustaceans known as a "Nauplius" (see tail-piece to the present chapter) in having (what no other wheel animalcule has) great hollow paired limbs moved by striated muscular fibre, carrying fringed hairs only known before in Crustaceans (crabs, shrimps and water fleas), and striking the water violently just as do those of the Nauplius. And yet all the while it has on its head a pair of large ciliated wheels which serve it just as do those of the common Rotifer. No Crustacean, young or old, has this "wheel-apparatus" nor any vibratile "cilia" on the surface of its body. Pedalion possesses an astounding "blend" of characters. Fig. 35 shows, besides the "paddles" or "legs" (of which two on the other side of the animal are not seen), the broad and large wheel-apparatus W (within which the right eye-spot r.e. is seen), and a little lobe (p) called the "chin" lying just below the mouth (m). The big leg (v.l.) and the pair on each side (l.l.1 and l.l.2), of which that on the right side only is seen, end in beautiful fringed hairs, which are only seen elsewhere in the Crustacea (water-fleas and others). Those on the lateral limbs and the great ventral limb (Fig. 36) are set in two groups of four on each side of the free end of the limb, whilst those on the dorsal leg (d.l.) are apparently not so numerous. I have corrected the drawings, Figs. 35 and 36, by reference to actual specimens kindly given to me by Mr. Rousselet.
Fig. 37.—The Rotifer Noteus quadricornis—to show its curious four-horned carapace—from which the wheel apparatus, wa, emerges in front, and the tail, t, behind; somewhat as the head and tail of a tortoise emerge from its protective "box" or carapace. The ridges on the horney covering of the Rotifer recall the horney plates of the tortoises and turtles.
The 500 different species of Wheel Animalcules or Rotifera differ from one another in the exact shape of the wheel-apparatus, in the jointing of the body and its general shape, and in the development, in some, of a hard skin or shell like a turtle's or tortoise's shell (Fig. 37) over that broadest region of the body in which in our Fig. 34, A, the stomach marked "St" is placed. They differ also in the shape of the gizzard's teeth, in the presence of paddles or legs (in Pedalion alone), and in the presence in some of longer or shorter projecting movable rods or bristles in pairs or in bunches. Many build for themselves tubular habitations of jelly or of hard cemented particles. They are all minute (from the ¹/₁₂ to the ¹/₅₀₀ in. in length). They are divided into five principal groups, which are (1) the crawlers, like the common Rotifer (Fig. 34), which can crawl like a leech and also swim freely by aid of their wheel-apparatus; (2) the naked free swimmers, which do not crawl, but move only by swimming; (3) the turtle-shelled free swimmers (Fig. 37) like the last, but provided with strong, often faceted, angular, and spike-bearing shells or "bucklers," from which head and wheel-apparatus project in front and narrow tail behind; (4) the rooted or fixed forms (Figs. 37 bis); these never swim when full grown, but each forms and inhabits a protective tube or case; (5) the skipping or darting forms. Of these there is only the Pedalion mirum (Figs. 35 and 36), which is quite unlike all the other wheel animalcules in having limbs like those of the minute water-fleas (Nauplius, Cyclops) which strike the water and are fringed with feather-like hairs.
The larval or young form of Crustacea known as "the Nauplius." This is the "Nauplius" of a kind of Prawn. The three pairs of branched limbs are well seen. Much magnified.