« НазадПродовжити »
abound on all sides. Even as regards the human species, there is a very generally admitted opinion that a new Those that did not so turn would much more easily become the prey of others (if they were vegetarians), or (if they were carnivorous animals) their prey would more readily perceive and avoid them. Here, then, it seems at first that we have the obvious action of natural selection. Yet, in fact, this faculty now appears to be rather the result of some obscure internal tendency, intensified, no doubt, by natural selection, but by no means due to the action of the latter on mere accidental minute variations. For in Siberia it is not only timid or predacious animals that tend to become light-coloured in winter, since Pallas tells us that such a phenomenon is observable in domestic cattle and in horses. In such animals, of course, this character cannot be due to natural selection, and therefore, as we must admit it in this case to be due to some spontaneous internal tendency, it immediately becomes probable that a similar tendency also exists in the other cases, and the onus probandi lies with him who denies that such is the case."—Novre Species Quadrupedum e Glirium ordine, 1778, p. 7, quoted by Mr. Darwin in “Descent of Man :” see vol. i. p. 282.
type has been developed in the United States, and this in about a couple of centuries only, and in a vast multitude of individuals of diverse ancestry. The instances here given, however, must suffice, though more could easily be adduced. It
may be well now to turn to groups presenting similar variations, not through, but independently of, geographical distribution, and, as far as we know, independently of conditions other than some peculiar nature and tendency (as
yet unexplained) common to members of such groups, which nature and tendency seem to induce them to vary in certain definite lines or directions which are different in different groups.
Thus with regard to the group of insects of which the walking leaf is a member, Mr. Wallace observes : 1 “The whole family of the Phasmidæ,
1 See “Natural Selection," p. 64.
or spectres, to which this insect belongs, is more or less imitative, and a great number of the species are called 'walking-stick insects,' from their singular resemblance to twigs and branches."
Again, Mr. Wallace 1 tells us of as many as four kinds of orioles, which birds mimic, more or less, four species of
a genus of honey-suckers, the weak orioles finding their profit in being mistaken by certain birds of prey for the strong, active, and gregarious honey-suckers. Now many other birds would be benefited by similar mimicry, which is none the less confined, in this part of the world, to the
It is true that the absence of mimicry in "Malay Archipelago," vol. ii. p. 150 ; and “ Natural Selection,” p. 104.
other forms may be explained by their possessing some other (as yet unobserved) means of preservation; but it is nevertheless remarkable, not so much that one species should mimic, as that no less than four should do so in different ways and degrees, all these four belonging to one and the same genus.
THE RED BIRD OF PARADISE.