Mimicry is a special branch of the study of adaptation. The term has sometimes been used loosely to include cases where an animal, most frequently an insect, bears a strong and often most remarkable resemblance to some feature of its inanimate surroundings. Many butterflies with wings closed are wonderfully like dead leaves; certain spiders when at rest on a leaf look exactly like bird-droppings; "looper" caterpillars simulate small twigs; the names of the "stick-" and "leaf-" insects are in themselves an indication of their appearance. Such cases as these, in which the creature exhibits a resemblance to some part of its natural surroundings, should be classified as cases of "protective resemblance" in contradistinction to mimicry proper. Striking examples of protective resemblance are abundant, and though we possess little critical knowledge of the acuity of perception in birds and other insect feeders it is plausible to regard the resemblances as being of definite advantage in the struggle for existence. However, it is with mimicry and not with protective coloration in general that we are here directly {9} concerned, and the nature of the phenomenon may perhaps best be made clear by a brief account of the facts which led to the statement of the theory. In the middle of last century the distinguished naturalist, H. W. Bates, was engaged in making collections in parts of the Amazon region. He paid much attention to butterflies, in which group he discovered a remarkably interesting phenomenon[3]. Among the species which he took were a large number belonging to the group Ithomiinae, small butterflies of peculiar appearance with long slender bodies and narrow wings bearing in most cases a conspicuous pattern (cf. Pl. X, fig. 7). When Bates came to examine his catch more closely he discovered that among the many Ithomiines were a few specimens very like them in general shape, colour, and markings, but differing in certain anatomical features by which the Pierinae, or "whites," are separated from other groups. Most Pierines are very different from Ithomiines. It is the group to which our common cabbage butterfly belongs and the ground colour is generally white. The shape of the body and also of the wings is in general quite distinct from what it is in the Ithomiines. Nevertheless in these particular districts certain of the species of Pierines had departed widely from what is usually regarded as their ancestral pattern (Pl. X, fig. 1) and had come to resemble very closely the far more abundant Ithomiines among whom they habitually flew (cf. Pl. X, figs. 2 and 3). To {10} use Bates' term they "mimicked" the Ithomiines, and he set to work to devise an explanation of how this could have come about. The _Origin of Species_ had just appeared and it was natural that Bates should seek to interpret this peculiar phenomenon on the lines there laid down. How was it that these Pierines had come to depart so