56 GENERAL PRINCIPLES OF ZOOLOGY. 



are one species of butterfly, several flies, and numerous 

 beetles. But most interesting are the cases of sympathetic 

 coloration and of mimicry, and finally, the appearance of 

 sexual characters as the result of sexual selection. 



Sympathetic Coloration. Very often, in regions which 

 have an entirely or preponderatingly uniform coloring, the 

 coat of the animals is distinguished by the same or at least 

 by a very similar coloring; this phenomenon is called sym- 

 pathetic coloration. Inhabitants of regions of snow are col- 

 ored white, desert animals have the pale yellow color of the 

 desert, animals which live at the surface of clear water are 

 transparent like crystal ; representatives of the most diverse 

 animal branches show the same phenomenon. The advan- 

 tage connected therewith scarcely needs an explanation. 

 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 which makes in either of these directions, and in 

 the course of many generations these advantages increase. 



Mimicry is referable to the same principle, except that 

 the imitation is not here limited to the color, but also in- 

 fluences form and marking. Exceedingly often parts of 

 plants are imitated, sometimes leaves, sometimes stems. 

 Certain diurnal butterflies with beautifully colored upper 

 surfaces of the wings 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 their under sides comes into 

 play and conceals the array of color on the upper side. 

 The parts are so laid over one another that the whole 

 takes on a leaflike form, and certain markings heighten the 

 imitation of the nervation of the leaf. Among the numer- 

 ous species of leaf-butterflies there are different grades of 

 completeness of mimicry ; in many even the damage done by 

 insects is imitated ; in others the form and marking are still 

 incompletely leaflike, the marking being the first to come 



