complete."[1] Mr. Wallace travelled both in the western and eastern tropics. The late Prof. Drummond records similar impressions in Africa:—"On finding one of these insects, I have often cut a small branch from an adjoining tree, and laid the two side by side for comparison; and when both are partly concealed by the hands so as to show only the part of the insect's body which is free from limbs, it is impossible to tell the one from the other. The very joints of the legs in these forms are knobbed to represent nodes, and the characteristic attitudes of the insects are all such as to sustain the deception."[2] Every writer, in fact, who approaches the subject of animal disguises, whether evolutionist or not, quotes these insects as one of the strongest illustrations he can find, and with ample warrant, for we may take these "Stick-insects" as affording a typical instance of what is understood as protective resemblance. The protection, however, cannot be complete, for Wallace found the stomachs of certain Cuckoos full of them.[3]
Now, it is a general postulate that this highly imitative and protected form is due to the action of "natural selection," acting on some incipient and original element of variation. As Mr. Bates observed:—"Natural selection having, from the first, favoured the species which offered variation in these parts, the tendency to variability has become perpetuated by inheritance."[4] Or, as Mr. Darwin put it:—"Assuming that an insect originally happened to resemble in some degree a dead twig or a decayed leaf, and that it varied slightly in many ways, then all the variations which rendered the insect at all more like any such object, and thus favoured its escape, would be preserved, whilst other variations would be neglected and ultimately lost; or, if they rendered the insect at all less like the imitated object, they would be eliminated."[5] We should therefore expect, if a perfect geological record could unfold the ancestry of these insects, to trace a gradual evolution of form for protective purposes under
- ↑ 'Contributions to the Theory of Natural Selection,' p. 64.
- ↑ 'Tropical Africa,' 4th edit. p. 173.
- ↑ 'Tropical Nature,' p. 93.—In North America "Walking-sticks (Diapheromera) are eaten by the Crow-Blackbird and two species of Cuckoos."—S. D. Judd (American 'Naturalist,' vol. xxxiii. p. 462).
- ↑ "Descriptions of Fifty-two New Species of Phasmidæ" (Trans. Linn. Soc. vol. xxv. p. 323).
- ↑ 'Origin of Species,' 6th ed. p. 182.
