HISTORY OF ZOOLOGY 37 



appreciate the methods and effects of the struggle for existence. Islands 

 in the midst of the ocean have a disproportionately large number of species 

 f wingless insects, because the flying forms are easily carried out to sea. 

 For example, on the Kerguelen Islands, remarkably exposed to storms, 

 the insects are wingless; among them one species of butterfly, several 

 flies, and numerous beetles. 



Sympathetic Coloration. Very often, in regions which have a pre- 

 vailing uniform color, the coat of the animals is distinguished by a similar 

 hue; this phenomenon is called sympatlietic coloration. Inhabitants of 

 regions of snow are white, desert animals have the pale yellow color of the 

 desert, animals which live at the surface of the sea are transparent; 

 representatives of the most diverse animal branches show the same phe- 

 nomenon. The advantages connected therewith scarcely need an expla- 

 nation. Every animal may have occasion to conceal himself from his 

 pursuers; or it may be his lot to approach his prey by stealth: he is much 

 better adapted for this the closer he resembles his surroundings. Natural 

 selection fixes every advantage in either of these directions, and in the 

 course of many generations these advantages increase. Among the most 

 interesting are the cases of sympathetic coloration, of mimicry and of the 

 development of secondary sexual characters as a result of sexual selection. 



Mimicry is referable to the same principle, except that the imitation 

 is not here limited to the color, but also influences form and marking. 

 Frequently parts of plants are imitated, sometimes leaves, sometimes 

 stems. Certain butterflies with the upper surfaces of the wings beauti- 

 fully colored escape their pursuers by the rapidity of their flight; if they 

 alight to rest, they are protected by their great similarity to the leaves of 

 the plants around which they chiefly fly. When the wings are folded over 

 the back, the dark coloring of the under sides comes into sight and the 

 color on the upper side is concealed. The parts are so. arranged that the 

 whole takes on a leaf-like form, and certain markings heighten the imita- 

 tion of the leaf (fig. n). Among the numerous species of leaf-butterflies 

 there are different grades of completeness of mimicry; in many even the 

 depredations of insects are imitated; in others the form and marking are 

 still incompletely leaf-like, the marking being the first to come into exis- 

 tence. Among the grasshoppers also there are imitations of leaves, like 

 the 'walking-leaf,' PliyUium sice (folium, P. scythe, while other nearly 

 related forms more or less completely approach the appearance of dried, 

 sometimes of thorny twigs (fig. 12, a and b). 



Very often insects are copied by other animals. Certain butterflies, 

 the Heliconias of warmer America, the Danaids of the Old World, fly 

 heavily in large swarms, clumsy and yet are unmolested by birds, because 



