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Show 138 DARWINISM CHAP. Instability of Non-adaptive Chamcters. One very weighty objection to the theory that specific characters can ever be wholly useless (or wholly unconnected with usefnl organs by correlation of growth) appears to have been overlooked by those who have muintaincd the frequency of such characters, and that is, their almoHt necessary instability. Darwin has remarked on the extreme variability of secondary sexual characters-such as the horm;, crests, plumes, etc., which are found in males only,-thc reason being, that, although of some usc, they are not of such direct and vital importance as those adapti vo characters on which the wellbeing and very existence of the animals depend. But in the case of wholly useless structurcR, 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 distribnten hairs, which evidently . ervc the purpose of mechanically raising the old skin by their rigidity and positio11. These hairs then may be designated as wsting hairs. 'l'hat they are destined and calculated for this end is evident to me from the fact established by lJr. Braun, that the casting of the shells of the river cray-fish 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 :tiHl Cartier have shown that these casting hairs- which serve the same purpose m two groups of animals o far apart in the systematic scale-after the casting, are partly transformed into the concentric stripes, sharp spikes, ridges, or warts which omament the outer edges of the skin-scales of reptiles or the carapace of crabs." 1 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 understall(l. During a recent discussion of this question in the pages of Nature, Mr. St. George Mivart adduces several examples of what he deems useless spccifi<' characters. Among them are the aborted index finger of the lemurinc Polio, antl the thumbless hands of Colobus and Ateles, the "life-saving action" of either of which he thinks incredible. These ca es suggest two remarks. In ihe fir t place, they involve generic, not spec{jic, characters; and the tlt n•c genera adduced are somewhat isolated, implying conshlerablc antiquity a111 l ihc exii.uction of mauy allied forms. This is important, hecau:c it afli))'(]s a111ple time for great changes of conditions since the structures in qu(•stion originated; antl without a knowledge of these changes we can never saf'(•ly assert tl1at any detail of structme could not have been n cful. In the set'ond place, all three are cases of aborted or rudimentary organs ; and these arc admitted to Le explainerl by non-use, leadincr to dimiuution of size, a furth er reduction being hmnght about by the act~n of the principle of economy 1 The Xatural Conditions of Existence as they affect Animal Life, p. 19. VI DIFFICULTIES AND OBJECTIONS 139 which are not rudiments of once useful organs, we cannot see what there is to ensure any amount of constancy or stability. One of the cases on which Mr. Romanes lays great stress in his paper on "Physiological Selection" (Jour·n. Linn. Soc., vol. xix. p. 384) is that of the fleshy appendages on the corners of the jaw of Normandy pigs and of some other breeds. But it is expressly stated that they are -not constant; they appear "frequently," or "occasionally," they are "not strictly inherited, for they occur or fail in animals of the same litter;" and they are not always symmetrical, sometimes appearing on one side of the face alone. Now whatever may be tho cause or explanation of these anomalous appendages they cannot be classed with "specific characters," the most essential features of which are, that they are symmetrical, of growth. But, when so re 1uced, the rudiment might he inconvenient or even hurtful, and then nat~1ral selection would aid in its complete abortion ; in other words, the abortion of the part would be 7/Sfju/, and would therefore be subject to the law of survival of the fittest. '!'he genera Ateles and Colobus are two ~f the most purely arboreal types of monkeys, and it is not difficult to concmve that the constant u e of the elongated fingers for climbing from tree .to tree, and catching on to brancl1es while making great leaps, might reqmre all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless. 'J'he ca:e of the Potto is more difficult, both because it is, presumably, a more ancicut type, and its actual lifehistory and habits are completely unknown. 'l'he:e cases are, therefore, not at all to the point as proving that positive specific characters-not mere rudintents 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 rl1eumatic monkeys, which might, nevertheles , get on very well under favourahle conditions. 'l'he struggle for existence, umler which all animals and plants ltave been developed, is intermittent, ancl exceed ingly irregu lar in it~ incirlence and severity. It i: most severe and fatal to tl1e young; but when an animal l1as once reached maturity, and especially when it has gained experience hy several years of an eventful existence, it may he able to maintain itself nnrler conditions which would be fatal to a yotmg and inexperienced creature of tl1e same species. 'l'he 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. I 1 See Nature, vol. xxxix. p. 127. |
