46 GENERAL PRINCIPLES OT ZOOLOGY. 



it in othor parts of the world. Several European species of thistle 

 have increased so enormously in the La Plata states that they liave 

 in places coniplotely crowded out the native plants. Another 

 European plant {Hypocltceris radicata) has become a weed, over- 

 running everything in New Zealand. Certain races of men, like 

 the Dravidian and Indian, die off to the same degree that other 

 races of men, like the Caucasian, Mongolian, and Negro, sjiread. 

 The more one attempts to explain that endlessly complicated web 

 of the relations of animals to one another, the relations of animals 

 to plants and to climatic conditions, as Darwin has done, so much 

 the more does he learn to appreciate the methods and results of 

 the struggle for existence. He will become conversant with many 

 interesting jjhenomena, formerly unintelligible, which immediately 

 find an explanation through this doctrine. Islands h'ing in the 

 midst of the ocean have a disproportionately large number of 

 species of 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 

 joermanent or prevailing uniform color, the coat of the animals is 

 distinguished by the same or at least by a similar hue; this 

 phenomenon is called si/mpatltetic 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 phenomenon. The advantages connected therewith 

 scarcely need 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 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 influences form 

 and marking. Frequently parts of plants are imitated, sometimes 

 leaves, sometimes stems. Certain butterflies with the upper sur- 

 faces of the wings beautifully 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 



