Secrets of Earth and Sea

by Sir E. Ray Lankester

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.