To take the cases of protective resemblance and mimicry. Darwin and Wallace have to start with a form something like the body mimicked, without giving any idea as to how that resemblance could arise. But with this key of memory we can open nature's treasure house much more fully. Look, for instance, at nocturnal insects; and one need not go further than the beetles (_Blatta_) in the kitchen, to see that they have a sense of need, and use it. Suddenly turn up the gas, and see the hurried scamper of the alarmed crowd. They are perfectly aware that danger is at hand. Equally well do they feel that safety lies in concealment; and while all the foraging party on the white floor are scuttling away into dark corners, the fortunate dweller on the hearth stands motionless beneath the shadow of the fire-irons; a picture of keen, intense excitement, with antennæ quivering with alertness. On the clean floor a careless girl has dropped a piece of flat coal, and on it beetles stand rigidly. They are as conscious as we are that the shadow, and the colour of the coal afford concealment, and we cannot doubt that they have become black from their sense of the protection they thus enjoy. They do not say, as Tom, the Water Baby, says, "I must be clean," but they know they must be black, and black they are. There is, then, clearly an effort to assimilate in hue to their surroundings, and the whole question is comparatively clear. Mr. Wallace, in commenting upon the butterfly (_Papilio nireus_)--which, at the Cape, in its chrysalis state, copies the bright hues of the vegetation upon which it passes its dormant phase--says that this is a kind of natural colour photography; thus reducing the action to a mere physical one. We might as well say the dun coat of the sportsman among the brown heather was acquired mechanically. Moreover, Wallace distinctly shows that when the larvæ are made to pupate on unnatural colours, like sky-blue or vermilion, the pupæ do not mimic the colour. There is no reason why "natural photography" should not copy this as well as the greens, and browns, and yellows. But how easy the explanation becomes when memory, the sense of need, and Butler's little "dose of reason," are admitted! For ages the butterfly has been acquainted with greens, and browns, and yellows, they are every day experiences; but it has no acquaintance with aniline dyes, and therefore cannot copy them. The moral of all this is that things become easy by repetition; that without experience nothing can be done well, and that the course of development is always in one direction, because the memory of the road traversed is not forgotten. [Illustration] [3] Evolution, Old and New, p. 346. [4] On a New Method of Expressing the Law of Specific Change. By A. Tylor. [5] Animals and Plants under Domestication, vol. ii., p. 350. [6] Animals and Plants under Domestication, vol. ii., p. 370.