Note DG — Chapter VI · ORGANIC EVOLUTION.

It would appear, from certain passages of his "Darwinism," that Mr. A. R. Wallace (e.g. p. 139, note) holds or held similar views. "The genera Ateles and Colobus," he says, "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." I should also have quoted Mr. Wallace's account of the twisting round of the eyes of flat-fishes—where he says that the constant repetition of the effort of twisting the eye towards the upper side of the head, when the bony structure is still soft and flexible, causes the eye gradually to move round the head till it comes to the upper side—had he not subsequently disclaimed this explanation (see Nature, vol. xl. p. 619). It is possible that Mr. Wallace, notwithstanding the words "constant use" in the passage I have quoted, merely intends to imply that the elongated fingers are of advantage in climbing, and are thus subject to natural selection, the thumb diminishing through economy of growth.