Imágenes de páginas
PDF
EPUB

forehead and ears, is thus thickly clothed; but it is a significant fact that the palms of the hands and the soles of the feet are quite naked, like the inferior surfaces of all four extremities in most of the lower animals.” 1

Genetic relationship and transmutation brought about by disuse would satisfactorily account for this state of things. When we are told that the sham-winged beetles inhabit small and exposed islands, it is easy to understand how "natural selection would continue slowly to reduce the organ, until it was rendered harmless and rudimentary.”

ATAVISM, or reversion to ancestral types, is another remarkable principle which has to be taken into the account. This is, after all, but one form of the principle of inheritance which so often forces itself upon our notice. It is, therefore, not necessary to adduce instances to support the fact. Most of us have had some experience of the manner in which plants and animals, allowed to run wild, always tend to return to wild types : and every one has daily opportunities of observing family features, family dispositions, family diseases, &c. What, then, is the meaning of such cases as the following? Why do pigeons of the most distinct race, when crossed, beget young which so frequently resemble in some, if not in all respects, the colour and markings of the rock-pigeon? How is it that a barb crossed with a spot, a nun with a tumbler, &c., themselves not having a single blue feather, produce blue offspring with the black bars on the tail feathers so peculiarly characteristic of the rock-pigeon ? If this means atavism, the tendency in fowls—of such pure breeds as the Game, Malay, Cochin, Dorking, and Bantam-to have black breasts, red backs, and frequently the identical plumage of the wild Gallus bankiva, indicates descent from the latter. The stripes on the shoulders and legs, so common in dun horses, mean descent from some remote parent like the quagga or zebra. The extraordinary growth of canine teeth occasionally found in man, as in

1 Darwin, The Descent of Man, vol. i. p. 26,

the case of the Naulette skull and others, points to an ancestry which is happily very distant. The abnormal occurrence of a double uterus in woman, interpreted in the same way, means reversion to the marsupial type; for this cannot be explained as a case of arrested development, since “no such stage is passed through during the ordinary development of the embryo."1

The method of CLASSIFICATION now universally adopted, whether by the opponents or advocates of evolution, is one and the same. What are the characters considered of most importance for the purpose ? Not “ those parts of the structure which determine the habits of life, and the general place of each being in the economy of nature." These are looked upon as merely "adaptive or analogical characters.” They may be mainly due to external conditions or so-called accident, and the true affinity may be completely hidden beneath them. Nor is the physiological importance of an organ any measure of its classificatory value; for the groups of animals possessed, for example, of such important organs as those for propelling the blood or for aerating it, or those for propagating the race, are so numerous that these characters are of subordinate value for classification. “Therefore," says Darwin, “we choose those characters which, so far as we can judge, are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed. Rudimentary structures on this view are as good, or even sometimes better, than other parts of the organisation. We care not how trifling a character may be, &c.; if it prevail throughout many and different species, especially those having very different habits of life, it assumes high value."2 And why? Not simply as a mark of resemblance, (we have far more obvious resemblances which we set aside), but as a mark of affinity-most important, because its presence cannot be accounted for on any hypothesis save that of its inheritance from a common 1 Descent of Man..

Origin of Species.

parent. Community of descent is the hidden bond which naturalists have been unconsciously seeking;" for, the natural system of classification is founded on descent with modification.

The remarkable instances of MIMICRY which were first seized upon by Mr. Bates, and have been so extensively illustrated by Mr. Wallace, seem to receive a simple explanation in the "survival of the fittest.” Indeed, Mr. Wallace's theorem, that “those races having colours best adapted to concealment from their enemies would invariably survive the longest,” is too self-evident to be disputed. Hares, partridges, grouse, ptarmigan, &c., are all protected by the resemblance of their own colours to those of their habitats. The sole or founder, lying at the bottom of the water, is identical in colour with the sand; while its under surface, as is that of all fish, is a bluish white, which serves admirably for protection against enemies when swimming beneath, and looking upwards toward the light. Mr. Bates and Mr. Wallace give scores of examples of imitation which serve the purpose of concealment. “The most wonderful and undoubted case of protective resemblance in a butterfly I have ever seen is that of the common Indian Kallima inachis, and its Malayan ally Kallima paralekta."1 These insects are described as exactly resembling the dead or decaying leaves amongst which they settle. “We come now to a still more extraordinary part of the imitation, for we find representations of the leaves in every stage of decay, variously blotched and mildewed and pierced with holes, and in many cases irregularly covered with powdery black dots gathered into patches and spots, so closely resembling the various kinds of minute fungi that grow on dead leaves that it is impossible to avoid thinking, at first sight, that the butterflies themselves have been attacked by real fungi.” The phyllium or“ walking leaf” is another. The “walking-stick insect” is another. “Some of these are a foot long and as thick as one's finger, and their whole colouring, form, rugosity, and the arrangement of the head, legs, and antennæ are such as to render them absolutely identical in appearance with dead sticks."

1 Wallace, Natural Selection, p. 59.

Another class of imitations to which Mr. Bates gave the name of “mimicry," is the likeness assumed by innoxious insects to others which are unpalatable or hurtful to insectivorous animals. There are bee-like, wasp-like, ichneumon-like, insects which are nothing but harmless moths. Mr. Bates discovered a beetle so like a wasp in form and colour that even he was at first afraid to touch it. The same distinguished naturalist describes a large caterpillar to outward appearances like a snake. “The three segments behind the head were dilatable at the will of the insect, and had on each side a large black pupillated spot which resembled the eye of the reptile. Moreover, it resembled a poisonous viper, not a harnıless species of snake, as was proved by the imitation of keeled scales on the crown, produced by the recumbent feet as the caterpillar threw itself backwards.1

Mr. Wallace has cited a number of instances to show that the dull colours of female birds are for the purpose of concealment during incubation. His case is immensely strengthened by the exceptions to the rule. When the plumage of the female is bright, she has either a domed nest or else builds in hidden places. When the plumage of the male is less conspicuous than that of the female, the male hatches the eggs instead of the female.

Darwin is of opinion that adornment and beauty of colour in animals is due to sexual selection. This view is countenanced by the habit of display in the males, so notable in peacocks; also by the brighter plumage or increased beauty of the male during the breeding season; and further by the decided preference evinced sometimes by the females in mating. Probably both these agencies are instrumental in producing the one effect. For although

i Ubi supra, p. 99.

the ends of concealment would be served by the dull plumage of the female, the greater safety of the male would not promote brilliant colouring.

With respect to the showiness of some caterpillars, butterflies, &c., it has been constantly observed that such insects are for some reason distasteful to other animals.

The brightness of flowers serves to attract the insects by whose instrumentality they are fecundated. I have come to this conclusion," says Darwin, “ from finding it an invariable rule, that when a flower is fertilised by the wind it never has a gaily-coloured corolla.”

We have now run through the stock arguments to be found in the best modern works on Evolution. It will be our task in the next few letters to consider some of the objections which the doctrines here set forth have given rise to.

VOL. I.

т

« AnteriorContinuar »