[42] See Kerner's Flowers and their Unbidden Guests for numerous other structures and peculiarities of plants which are shown to be adaptive and useful.
[43] Nature, vol. xx. p. 603.
[44] Nature, vol. xxxviii. p. 328.
[45] A very remarkable illustration of function in an apparently useless ornament is given by Semper. He says, "It is known that the skin of reptiles encloses the body with scales. These scales are distinguished by very various sculpturings, highly characteristic of the different species. Irrespective of their systematic significance they appear to be of no value in the life of the animal; indeed, they are viewed as ornamental without regard to the fact that they are microscopic and much too delicate to be visible to other animals of their own species. It might, therefore, seem hopeless to show the necessity for their existence on Darwinian principles, and to prove that they are physiologically active organs. Nevertheless, recent investigations on this point have furnished evidence that this is possible.
"It is known that many reptiles, and above all the snakes, cast off the whole skin at once, whereas human beings do so by degrees. If by any accident they are prevented doing so, they infallibly die, because the old skin has grown so tough and hard that it hinders the increase in volume which is inseparable from the growth of the animal. The casting of the skin is induced by the formation on the surface of the inner epidermis, of a layer of very fine and equally distributed hairs, which evidently serve the purpose of mechanically raising the old skin by their rigidity and position. These hairs then may be designated as casting hairs. That they are destined and calculated for this end is evident to me from the fact established by Dr. Braun, that the casting of the shells of the river crayfish is induced in exactly the same manner by the formation of a coating of hairs which mechanically loosens the old skin or shell from the new. Now the researches of Braun and Cartier have shown that these casting hairs—which serve the same purpose in two groups of animals so far apart in the systematic scale—after the casting, are partly transformed into the concentric stripes, sharp spikes, ridges, or warts which ornament the outer edges of the skin-scales of reptiles or the carapace of crabs." [The Natural Conditions of Existance as they affect Animal Life, p. 19.] Professor Semper adds that this example, with many others that might be quoted, shows that we need not abandon the hope of explaining morphological characters on Darwinian principles, although their nature is often difficult to understand.
During a recent discussion of this question in the pages of Nature, Mr. St. George Mivart adduces several examples of what he deems useless specific characters. Among them are the aborted index finger of the lemurine Potto, and the thumbless hands of Colobus and Ateles, the "life-saving action" of either of which he thinks incredible. These cases suggest two remarks. In the first place, they involve generic, not specific, characters; and the three genera adduced are somewhat isolated, implying considerable antiquity and the extinction of many allied forms. This is important, because it affords ample time for great changes of conditions since the structures in question originated; and without a knowledge of these changes we can never safely assert that any detail of structure could not have been useful. In the second place, all three are cases of aborted or rudimentary organs; and these are admitted to be explained by non-use, leading to diminution of size, a further reduction being brought about by the action of the principle of economy of growth. But, when so reduced, the rudiment might be inconvenient or even hurtful, and then natural selection would aid in its complete abortion; in other words, the abortion of the part would be useful, and would therefore be subject to the law of survival of the fittest. The genera Ateles and Colobus are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless. The case of the Potto is more difficult, both because it is, presumably, a more ancient type, and its actual life-history and habits are completely unknown. These cases are, therefore, not at all to the point as proving that positive specific characters—not mere rudiments characterising whole genera—are in any case useless.
Mr. Mivart further objects to the alleged rigidity of the action of natural selection, because wounded or malformed animals have been found which had evidently lived a considerable time in their imperfect condition. But this simply proves that they were living under a temporarily favourable environment, and that the real struggle for existence, in their case, had not yet taken place. We must surely admit that, when the pinch came, and when perfectly formed stoats were dying for want of food, the one-footed animal, referred to by Mr. Mivart, would be among the first to succumb; and the same remark will apply to his abnormally toothed hares and rheumatic monkeys, which might, nevertheless, get on very well under favourable conditions. The struggle for existence, under which all animals and plants have been developed, is intermittent, and exceedingly irregular in its incidence and severity. It is most severe and fatal to the young; but when an animal has once reached maturity, and especially when it has gained experience by several years of an eventful existence, it may be able to maintain itself under conditions which would be fatal to a young and inexperienced creature of the same species. The examples adduced by Mr. Mivart do not, therefore, in any way impugn the hardness of nature as a taskmaster, or the extreme severity of the recurring struggle for existence. [See Nature, vol. xxxix. p. 127.]
[46] Origin of Species, p. 72.
[47] Darwin's latest expression of opinion on this question is interesting, since it shows that he was inclined to return to his earlier view of the general, or universal, utility of specific characters. In a letter to Semper (30th Nov. 1878) he writes: "As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have, of late years, been confined. Therefore it seems to me rather rash to consider slight differences between representative species, for instance, those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection" (Life of Darwin, vol. iii. p. 161).
[48] See Variation of Animals and Plants, vol. i. p. 86.