The Effect of External Influences
We have already seen that Lamarck held as a part of his doctrine of transformation that the changes in the external world, the environment, bring about, directly, changes in the organism, and he believed that all plants and many of the lower animals have evolved as the result of a reaction of this sort. This idea did not originate with Lamarck, however, since before him Buffon had advanced the same hypothesis, and there cannot be much doubt that Lamarck borrowed from his patron, Buffon, this part of his theory of evolution.
This idea of the influence of the external world as a factor inducing changes in the organism has come, however, to be associated especially with the name of Geoffroy Saint-Hilaire, whose period of activity, although overlapping, came after that of Lamarck. The central idea of Geoffroy’s view was that species of animals and plants undergo change as the environment changes; and it is important to note, in passing, that he did not suppose that these changes were always for the benefit of the individual, i.e. they were not always adaptive. If they were not, the forms became extinct. So long as the conditions remain constant, the species remains constant; and he found an answer in this to Cuvier’s argument, in respect to the similarity between the animals living at present in Egypt and those discovered embalmed along with mummies at least two thousand years old. Geoffroy Saint-Hilaire said, that since the climatic conditions of Egypt had remained exactly the same during all these years, the animals of Egypt would also have remained unchanged.
Geoffroy’s views were largely influenced by his studies in systematic zoology and by his conception of a unity of plan running through the entire animal kingdom. His study of embryology and paleontology had led him to believe that present forms have descended from other organisms living in the past, and in this connection his discovery of teeth in the jaws of the embryo of the baleen whale and also his discovery of the embryonic dental ridges in the upper and in the lower jaws of birds, were used with effect in supporting the theory of change or evolution. Lastly, his remarkable work in the study of abnormal forms prepared the way for his conception of sudden and great changes, which he believed organisms capable of undergoing. He went so far in fact, in one instance, as to suppose that it was not impossible that a bird might have issued fully equipped from the egg of a crocodile. Such an extreme statement, which seems to us nowadays only laughable, need not prejudice us against the more moderate parts of his speculation.
His study of the fossil gavials found near Caen led him to believe that they are quite distinct from living crocodiles. He asked whether these old forms may not represent a link in the chain that connects, without interruption, the older inhabitants of the earth with animals living at the present time. Without positively affirming that this is the case, he did not hesitate to state that a transformation of this sort seemed possible to him. He said: “I think that the process of respiration constitutes an acquirement so important in the ‘disposition’ of the forms of animals, that it is not at all necessary to suppose that the surrounding respiratory gases become modified quickly and in large amount in order that the animal may become slowly modified. The prolonged action of time would ordinarily suffice, but if combined with a cataclysm, the result would be so much the better.”
He supposed that in the course of time respiration becomes difficult and finally impossible as far as certain systems of organs are concerned. The necessity then arises and creates another arrangement, perfecting or altering the existing structures. Modifications, fortunate or fatal, are created which through propagation are continued, and which, if fortunate, influence all the rest of the organization. But if the modifications are injurious to the animals in which they have appeared, the animals cease to exist, and are replaced by others having a different form, and one suited to the new circumstances.
The comparison between the stages of development of the individual and the evolution of the species was strongly impressed on the mind of Geoffroy. He says: “We see, each year, the spectacle of the transformation in organization from one class into another. A batrachian is at first a fish under the name of a tadpole, then a reptile (amphibian) under that of a frog.” “The development, or the result of the transformation, is brought about by the combined action of light and of oxygen; and the change in the body of the animal takes place by the production of new blood-vessels, whose development follows the law of the balancing of organs, in the sense, that if the circulating fluids precipitate themselves into new channels there remains less in the old vessels.” By preventing tadpoles from leaving the water, Geoffroy claims that it has been shown that they can be prevented from changing into frogs. The main point that Geoffroy attempts to establish is no doubt fairly clear, but the way in which he supposes the change to be effected is not so clear, and his ideas as to the way in which new change may be perpetuated in the next generation are, from our more modern point of view, extremely hazy. It is perhaps not altogether fair to judge his view from the standpoint of the origin of adaptive structures, but rather as an attempt to explain the causes that have brought about the evolution of the organic world.
During the remainder of the nineteenth century there accumulated a large number of facts in relation to the action of the external conditions in bringing about changes in animals and plants. Much of this evidence is of importance in dealing with the question of the origin of organic adaptation.
The first class of facts in this connection is that of geographical variation in animals and plants. It will be impossible here to do more than select some of the most important cases. De Varigny, in his book on “Experimental Evolution,” has brought together a large number of facts of this kind, and from his account the following illustrations have been selected. He says: “When the small brown honey-bee from High Burgundy is transported into Bresse—although not very distant—it soon becomes larger and assumes a yellow color; this happens even in the second generation.” It is also pointed out that the roots of the beet, carrot, and radish are colorless in their wild natural state, but when brought under cultivation they become red, yellow, etc. Vilmorin has noted that the red, yellow, and violet colors of carrots appear only some time after the wild forms have been brought under cultivation. Moquin-Tandon has seen “gentians which are blue in valleys become white on mountains.” Other cases also are on record in which the colors of a plant are dependent on external conditions.
The sizes of plants and animals are also often directly traceable to certain external conditions; the change is generally connected with the amount of food obtainable. “Generally speaking,” De Varigny says, “insular animals are smaller than their continental congeners. In the Canary Islands the oxen of one of the smallest islands are smaller than those on the others, although all belong to the same breed, and the horses are also smaller, and the indigenous inhabitants are in the same case, although belonging to a tall race. It would seem that in Malta elephants were very small,—fossil elephants, of course,—and that during the Roman period the island was noted for a dwarf breed of dogs, which was named after its birthplace, according to Strabo. In Corsica, also, horses and oxen are very small, and Cervus corsicanus, the indigenous deer, is quite reduced in dimensions; ... and lastly, the small dimensions of the Falkland horses—imported from Spain in 1764—are familiar to all. The dwarf rabbits of Porto Santo described by Darwin may also be cited as a case in point.”
These facts, interesting as they are, will, no doubt, have to be more carefully examined before the evidence can have great value, for it is not clear what factor or factors have produced the decrease in size of these animals.
The following cases show more clearly the immediate effect of the environment: “Many animals, when transferred to warm climates, lose their wool, or their hairy covering is much reduced. In some parts of the warmer regions of the earth, sheep have no wool, but merely hairs like those of dogs. Similarly, as Roulin notices, poultry have, in Columbia, lost their feathers, and while the young are at first covered with a black and delicate down, they lose it in great part as they grow, and the adult fowls nearly realize Plato’s realistic description of man—a biped without feathers. Conversely, many animals when transferred from warm to cold climates acquire a thicker covering; dogs and horses, for instance, becoming covered with wool.”
A number of kinds of snails that were supposed to belong to different species have been found, on further examination, to be only varieties due to the environment. “Locard has discovered through experiments that L. turgida and elophila are mere varieties—due to environment—of the common Lymnæa stagnalis.” He says, “These are not new species, but merely common aspects of a common type, which is capable of modification and of adaptation according to the nature of the media in which it has to live.” It has also been shown by Bateson that similar changes occur in Cardium edule, and other lamellibranchs are known to vary according to the nature of the water in which they live.
In regard to plants, the influence of the environment has long been known to produce an effect on the form, color, etc., of the individuals. “The common dandelion (Taraxacum densleonis) has in dry soil leaves which are much more irregular and incised, while they are hardly dentate in marshy stations, where it is called Taraxacum palustre.
“Individuals growing near the seashore differ markedly from those growing far inland. Similarly, species such as some Ranunculi, which can live under water as well as in air, exhibit marked differences when considered in their different stations, as is well known to all. These differences may be important enough to induce botanists to believe in the existence of two different species when there is only one.”
An interesting case is that of Daphnia rectirostris, a small crustacean living sometimes in fresh water, at other times in water containing salt and also in salt lakes. There are two forms, corresponding to the conditions under which they live, and it is said that the differences are of a kind that suffice to separate species from each other. In another crustacean, Branchipus ferox, the form differs in a number of points, according to whether it lives in salt or in fresh water. Schmankewitsch says that, had he not found all transitional forms, and observed the transformation in cultures, he would have regarded the two forms as separate species. The oft-quoted case of Artemia furnishes a very striking example of the influence of the environment. Artemia salina lives in water whose concentration varies between 5 and 12 degrees of saltness. When the amount of salt is increased to 12 degrees, the animal shows certain characteristics like those of Artemia milhausenii, which may live in water having 24 to 25 degrees of saltness. The form A. salina may be further completely changed into that of A. milhausenii by increasing the amount of salt to the latter amount.
Among domesticated animals and plants—a few instances of which have been already referred to—we find a large number of cases in which a change in the environment produces definite changes in the organism. Darwin has made a most valuable collection of facts of this kind in his “Animals and Plants under Domestication.” He believes that domesticated forms are much more variable than wild ones, and that this is due, in part, to their being protected from competition, and to their having been removed from their natural conditions and even from their native country. “In conformity with this, all our domesticated productions without exception vary far more than natural species. The hive-bee, which feeds itself, and follows in most respects its natural habits of life, is the least variable of all domesticated animals.... Hardly a single plant can be named, which has long been cultivated and propagated by seed, that is not highly variable.” “Bud-variation ... shows us that variability may be quite independent of seminal reproduction, and likewise of reversion to long-lost ancestral characters. No one will maintain that the sudden appearance of a moss-rose on a Provence rose is a return to a former state, ... nor can the appearance of nectarines on peach trees be accounted for on the principle of reversion.” It is said that bud-variations are also much more frequent on cultivated than on wild plants.
Darwin adds: “These general considerations alone render it probable that variability of every kind is directly or indirectly caused by changed conditions of life. Or to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability.”
In some cases it has been observed that, in passing from one part of a continent to another, many or all of the forms of the same group and even of different groups change in the same way. Allen’s account of the variations in North American birds and mammals furnishes a number of striking examples of this kind of change. He finds that, as a rule, the birds and mammals of North America increase in size from the south northward. This is true, not only for the individuals of the same species, but generally the largest species of each genus are in the north. There are some exceptions, however, in which the increase in size is in the opposite direction. The explanation of this is that the largest individuals are almost invariably found in the region where the group to which the species belongs receives its greatest numerical development. This Allen interprets as the hypothetical “centre of distribution of the species,” which is in most cases doubtless also its original centre of dispersal. If the species has arisen in the north, then the northern forms are the largest; but if it arose in the south, the reverse is the case. Thus, most of the species of North America that live north of Mexico are supposed to have had a northern origin, as shown by the circumpolar distribution of some of them and by the relationship of others to Old World species; and in these the largest individuals of the species of a genus are northern. Conversely, in the exceptional cases of increase in size toward the south, it can be shown that the forms have probably had a southern origin.
The Canidæ (wolves and foxes) have their largest representatives, the world over, in the north. “In North America the family is represented by six species, the smallest of which (speaking generally) are southern and the largest northern.” The three species that have the widest ranges (the gray wolf, the common fox, and the gray fox) show the most marked differences in size. The skull, for instance, of “the common wolf is fully one-fifth larger in the northern parts of British America and Alaska than it is in northern Mexico, where it finds the southern limit of its habitat. Between the largest northern skull and the largest southern skull there is a difference of about thirty-five per cent of the mean size. Specimens from the intermediate region show a gradual intergradation between the extremes, although many of the examples from the upper Missouri country are nearly as large as those from the extreme north.” The common fox is about one-tenth larger, on the average, in Alaska than it is in New England. The gray fox, whose habitat extends from Pennsylvania southward to Yucatan, has an average length of skull of about five inches in the north, and less than four in Central America—about ten per cent difference.
The Felidæ, or cats, “reach their greatest development as respects both the number and the size of the species in the intertropical regions. This family has sent a single typical representative, the panther (Felis concolor), north of Mexico, and this ranges only to about the northern boundary of the United States. The other North American representatives of the family are the lynxes, which in some of their varieties range from Alaska to Mexico.” Although they vary greatly in different localities in color and in length and texture of pelage, they do not vary as to the size of their skulls. On the other hand the panther (and the ocelots) greatly increases in size southward, “or toward the metropolis of the family.”
Other carnivora that increase in size northward are the badger, the marten, the fisher, the wolverine, and the ermine, which are all northern types.
Deer are also larger in the north; in the Virginia deer the annually deciduous antlers of immense size reach their greatest development in the north. The northern race of flying squirrels is one-half larger than the southern, “yet the two extremes are found to pass so gradually one into the other, that it is hardly possible to define even a southern and a northern geographical race.” The species ranges from the arctic regions to Central America.
In birds also similar relations exist, but there is less often an increase in size northward. In species whose breeding station covers a wide range of latitude, the northern birds are not only smaller, but have quite different colors, as is markedly the case in the common quail, the meadow-lark, the purple grackle, the red-winged blackbird, the flicker, the towhee bunting, the Carolina dove, and in numerous other species. The same difference is also quite apparent in the blue jay, the crow, in most of the woodpeckers, in the titmice, numerous sparrows, and several warblers and thrushes. The variation often amounts to from ten to fifteen per cent of the average size of the species.
Allen also states that certain parts of the animal may vary proportionately more than the general size, there being an apparent tendency for peripheral parts to enlarge toward the warmer regions, i.e. toward the south. “In mammals which have the external ears largely developed—as in the wolves, foxes, some of the deer, and especially the hares—the larger size of this organ in southern as compared with northern individuals of the same species, is often strikingly apparent.” It is even more apparent in species inhabiting open plains. The ears of the gray rabbit of the plains of western Arizona are twice the size of those of the Eastern states.
In birds the bill especially, but also the claws and tail, is larger in the south. In passing from New England southward to Florida the bill in slender-billed forms becomes larger, longer, more attenuated, and more decurved; while in short-billed forms the southern individuals have thicker and larger bills, although the birds themselves are smaller.
The remarkable changes and gradations of color in birds in different parts of North America are very instructive, and the important results obtained by American ornithologists form an interesting chapter in zoology. The evidence would convince the most sceptical of the difficulty of distinguishing between Linnæan species. It is not surprising to find in this connection a leading ornithologist exclaiming, “if there really are such things as species.” The differences here noted are mainly from east to west. We may briefly review here a few striking cases selected from Coues’s “Key to North American Birds.”
The flicker, or golden-winged woodpecker (Colaptes auratus), has a wide distribution in eastern North America. It is replaced in western North America (from the Rocky Mountains to the Pacific) by C. mexicanus. In the intermediate regions, Missouri and the Rocky Mountain region, the characters of the two are blended in every conceivable degree in different specimens. “Perhaps it is a hybrid, and perhaps it is a transitional form, and doubtless there are no such things as species in Nature.... In the west you will find specimens auratus on one side of the body, mexicanus on the other.” There is a third form, C. chrysoides, with the wings and tail as in auratus, and the head as in mexicanus, that lives in the valley of the Colorado River, Lower California, and southward.
In regard to the song-sparrow (Melospiza), Coues writes: “The type of the genus is the familiar and beloved song-sparrow, a bird of constant characters in the east, but in the west is split into numerous geographical races, some of them looking so different from typical fasciata that they have been considered as distinct species, and even placed in other genera. This differentiation affects not only their color, but the size, relative proportions of parts, and particularly the shape of the bill; and it is sometimes so great, as in the case of M. cinerea, that less dissimilar looking birds are commonly assigned to different genera. Nevertheless the gradation is complete, and affected by imperceptible degrees.... The several degrees of likeness and unlikeness may be thrown into true relief better by some such expressions as the following, than by formal antithetical phrases: (1) The common eastern bird commonly modified in the interior into the duller colored (2) fallax. This in the Pacific watershed, more decidedly modified by deeper coloration,—broader black streaks in (3) hermanni, with its diminutive local race (4) samuelis, and more ruddy shades in (5) guttata northward, increasing in intensity with increased size in (6) rafina. Then the remarkable (7) cinerea, insulated much further apart than any of the others. A former American school would probably have made four ‘good species,’ (1) fasciata, (2) samuelis, (3) rafina, (4) cinerea.”
Somewhat similar relations are found in three other genera of finches. Thus Passerella is “imperfectly differentiated”; Junco is represented by one eastern species, but in the west the stock splits up into numerous forms, “all of which intergrade with each other and with the eastern bird. Almost all late writers have taken a hand at Junco, shuffling them about in the vain attempt to decide which are ‘species’ and which ‘varieties.’ All are either or both, as we may elect to consider them.” In the distribution of the genus Pipilo similar relations are found. There is an eastern form much more distinct from the western forms than these are from each other.
Finally may be mentioned the curious variations in screech-owls of the genus Scops. This owl has two strikingly different plumages—a mottled gray and a reddish brown, which, although very distinct when fully developed, yet “are entirely independent of age, season, or sex.” There is an eastern form, Scops asio, that extends west to the Rocky Mountains. There is a northwestern form, S. kennicotti, which in its red phase is quite different from S. asio, but in its gray plumage is very similar. The California form, S. benderii, is not known to have a red phase, and the gray phase is quite different from that of S. asio, but like the last form. The Colorado form, S. maxwellæ, has no red phase, “but on the contrary the whole plumage is very pale, almost as if bleached, the difference evident in the nestlings even.” The Texas form, S. maselli, has both phases, and is very similar to S. asio. The Florida form is smaller and colored like S. asio. The red phase is the frequent, if not the usual, one. The flammulated form, S. fiammula, is “a very small species, with much the general aspect of an ungrown S. asio.” This is the southwestern form, easily distinguished on account of its small size and color from the other forms.
These examples might be greatly increased, but they will suffice, I think, to convince one of the difficulty of giving a sharp definition to “species.” The facts speak strongly in favor of the transmutation theory, and show us how a species may become separated under different conditions into a number of new forms, which would be counted as new different species, if the intermediate forms were exterminated.
In discussing the nature of the changes that bring about variability, Darwin remarks: “From a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have varied. We see this with the many domestic races of quadrupeds and birds belonging to different orders, with goldfish and silkworms, with plants of many kinds, raised in various quarters of the world. In the deserts of northern Africa the date-palm has yielded thirty-eight varieties; in the fertile plains of India it is notorious how many varieties of rice and of a host of other plants exist; in a single Polynesian island, twenty-four varieties of the breadfruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives. The mulberry tree of India and Europe has yielded many varieties serving as food for the silkworm; and in China sixty-three varieties of the bamboo are used for various domestic purposes. These facts, and innumerable others which could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability—different changes acting on different organisms.”
Darwin thinks that a change in climate alone is not one of the potent causes of variability, because the native country of a plant, where it has been longest cultivated, is where it has oftenest given rise to the greatest number of varieties. He thinks it also doubtful that a change in food is an important source of variability, since the domestic pigeon has varied more than any other species of fowl, yet the food has been always nearly the same. This is also true for cattle and sheep, whose food is probably much less varied in kind than in the wild species.
Another point of interest is raised by Darwin. He thinks, as do others also, that the influence of a change in the conditions is cumulative, in the sense that it may not appear until the species has been subjected to it for several generations. Darwin states that universal experience shows that when new plants are first introduced into gardens they do not vary, but after several generations they will begin to vary to a greater or less extent. In a few cases, as in that of the dahlia, the zinnia, the Swan River daisy, and the Scotch rose, it is known that the new variations only appeared after a time. The following statement by Salter is then quoted, “Every one knows that the chief difficulty is in breaking through the original form and color of the species, and every one will be on the lookout for any natural sport, either from seed or branch; that being once obtained, however trifling the change may be, the result depends on himself.” Jonghe is also quoted to the effect that “there is another principle, namely, that the more a type has entered into a state of variation, the greater is the tendency to continue doing so, and the more it has varied from the original type, the more is it disposed to vary still further.” Darwin also quotes with approval the opinion of the most celebrated horticulturist of France, Vilmorin, who maintained that “when any particular variation is desired, the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction; for the fixed character of the species being once broken, the desired variation will sooner or later appear.”
Darwin also cites a few cases where animals have changed quite quickly when brought under domestication. Turkeys raised from the eggs of wild species lose their metallic tints, and become spotted with white in the third generation. Wild ducks lose their true plumage after a few generations. “The white collar around the neck of the mallard becomes much broader and more irregular, and white feathers appear in the duckling’s wings. They increase also in size of body.” In these cases it appears that several generations were necessary in order to bring about a marked change in the original type, but the Australian dingoes, bred in the Zoological Gardens, produced puppies which were in the first generation marked with white and other colors.
The following cases from De Varigny are also very striking. The dwarf trees from Japan, for the most part conifers, which may be a hundred years old and not be more than three feet high, are in part the result “of mechanical processes which prevent the spreading of the branches, and in part of a starving process which consists in cutting most roots and in keeping the plant in poor soil.”
As an example of the sudden appearance of a new variation the following case is interesting. A variety of begonia is recorded as having appeared quite suddenly at a number of places at the same time. In another case a narcissus which had met with adverse circumstances, and had then been supplied with a chemical manure in some quantity, began to bear double flowers.
Amongst animals the following cases of the appearance of sudden variations are pointed out by De Varigny. “In Paraguay, during the last century (1770), a bull was born without horns, although his ancestry was well provided with these appendages, and his progeny was also hornless, although at first he was mated with horned cows. If the horned and the hornless were met in fossil state, we would certainly wonder at not finding specimens provided with semi-degenerate horns, and representing the link between both, and if we were told that the hornless variety may have arisen suddenly, we should not believe it and we should be wrong. In South America also, between the sixteenth and eighteenth centuries the niata breed of oxen sprang into life, and this breed of bulldog oxen has thriven and become a new race. So in the San Paulo provinces of Brazil, a new breed of oxen suddenly appeared which was provided with truly enormous horns, the breed of franqueiros, as they are called. The mauchamp breed of sheep owes its origin to a single lamb that was born in 1828 from merino parents, but whose wool, instead of being curly like that of its parents, remained quite smooth. This sudden variation is often met with, and in France has been noticed in different herds.”
The ancon race of sheep originated in 1791 from a ram born in Massachusetts having short crooked legs and a long back. From this one ram by crossing, at first with common sheep, the ancon race has been produced. “When crossed with other breeds the offspring, with rare exception, instead of being intermediate in character, perfectly resemble either parent; even one of twins has resembled one parent and the second the other.”
Two especially remarkable cases remain to be described. These are the Porto Santo rabbit and the japanned peacock. Darwin has given a full account of both of these cases. “The rabbits which have become feral on the island of Porto Santo, near Madeira, deserve a fuller account. In 1418 or 1419 J. Gonzales Zarco happened to have a female rabbit on board which had produced young during the voyage, and he turned them all out on the island. These animals soon increased so rapidly that they became a nuisance, and actually caused the abandonment of the settlement. Thirty-seven years subsequently, Cada Mosto describes them as innumerable; nor is this surprising, as the island was not inhabited by any beast of prey, or by any terrestrial mammal. We do not know the character of the mother rabbit; but it was probably the common domestic kind. The Spanish peninsula, whence Zarco sailed, is known to have abounded with the common wild species at the most remote historical period; and as these rabbits were taken on board for food, it is improbable that they should have been of any peculiar breed. That the breed was well domesticated is shown by the doe having littered during the voyage. Mr. Wollaston, at my request, brought two of these feral rabbits in spirits of wine; and, subsequently, Mr. W. Haywood sent home three more specimens in brine and two alive. These seven specimens, though caught at different periods, closely resemble each other. They were full-grown, as shown, by the state of their bones. Although the conditions of life in Porto Santo are evidently highly favorable to rabbits, as proven by their extraordinarily rapid increase, yet they differ conspicuously in their small size from the wild English rabbit.... In color the Porto Santo rabbit differs considerably from the common rabbit; the upper surface is redder, and is rarely interspersed with any black or black-tipped hairs. The throat and certain parts of the under surface, instead of being pure white, are generally gray or leaden color. But the most remarkable difference is in the ears and tail. I have examined many fresh English rabbits, and the large collection of skins in the British Museum from various countries, and all have the upper surface of the tail and the tips of the ears clothed with blackish gray fur; and this is given in most works as one of the specific characters of the rabbit. Now in the seven Porto Santo rabbits the upper surface of the tail was reddish brown, and the tips of the ears had no trace of the black edging. But here we meet with a singular circumstance: in June, 1861, I examined two of these rabbits recently sent to the Zoological Gardens and their tails and ears were colored as just described; but when one of their dead bodies was sent to me in February, 1863, the ears were plainly edged, and the upper surface of the tail was covered with blackish gray fur, and the whole body was much less red; so that under the English climate this individual rabbit had recovered the proper color of its fur in rather less than four years.”
Another striking case of sudden variation is found in the peacock. It is all the more remarkable because this bird has hardly varied at all under domestication, and is almost exactly like the wild species living in India to-day. Darwin states: “There is one strange fact with respect to the peacock, namely, the occasional appearance in England of the ‘japanned’ or ‘black-shouldered’ kind. This form has lately been named, on the high authority of Mr. Slater, as a distinct species, viz. Pavo nigripennis, which he believes will hereafter be found wild in some country, but not in India, where it is certainly unknown. The males of these japanned birds differ conspicuously from the common peacock in the color of their secondary wing-feathers, scapulars, wing-coverts, and thighs, and are, I think, more beautiful; they are rather smaller than the common sort, and are always beaten by them in their battles, as I hear from the Hon. A. S. G. Canning. The females are much paler-colored than those of the common kind. Both sexes, as Mr. Canning informs me, are white when they leave the egg, and they differ from the young of the white variety only in having a peculiar pinkish tinge on their wings. These japanned birds, though appearing suddenly in flocks of the common kind, propagate their kind quite truly.”
In two cases, in which these birds had appeared quite suddenly in flocks of the ordinary kind, it is recorded that “though a smaller and weaker bird, it increased to the extinction of the previously existing breed.” Here we have certainly a remarkable case of a new species suddenly appearing and replacing the ordinary form, although the birds are smaller, and are beaten in their battles.
Darwin has given an admirably clear statement of his opinion as to the causes of variability in the opening paragraph of his chapter dealing with this topic in his “Animals and Plants.” Some authors, he says, “look at variability as a necessary contingent on reproduction, and as much an original law as growth or inheritance. Others have of late encouraged, perhaps unintentionally, this view by speaking of inheritance and variability as equal and antagonistic principles. Pallas maintained, and he has had some followers, that variability depends exclusively on the crossing of primordially distinct forms. Other authors attribute variability to an excess of food, and with animals, to an excess relatively to the amount of exercise taken, or again, to the effects of a more genial climate. That these causes are all effective is highly probable. But we must, I think, take a broader view, and conclude that organic beings, when subjected during several generations to any change whatever in their condition, tend to vary; the kind of variation which ensues depending in most cases in a far higher degree on the nature of the constitution of the being, than on the nature of the changed conditions.”
Most naturalists will agree, in all probability, with this conclusion of Darwin’s. The examples cited in the preceding pages have shown that there are several ways in which the organisms may respond to the environment. In some cases it appears to affect all the individuals in the same way; in other cases it appears to cause them to fluctuate in many directions; and in still other cases, without any recognizable change in the external conditions, new forms may suddenly appear, often of a perfectly definite type, that depart widely from the parent form.
For the theory of evolution it is a point of the first importance to determine which of these modes of variation has supplied the basis for evolution. Moreover, we are here especially concerned with the question of how adaptive variations arise. Without attempting to decide for the present between these different kinds of variability, let us examine certain cases in which an immediate and adaptive response to the environment has been described as taking place.