V NATURAL SELECTION 109 



other hand, seize their prey upon the ground, and the former 

 feed largely on reptiles and offal as well as on birds and 

 quadrupeds. Others have adopted fish as their chief food, 

 and the osprey snatches its prey from the water with as much 

 facility as a gull or a petrel ; while the South American 

 caracaras (Polyborus) have adopted the habits of vultures and 

 live altogether on carrion. In every great group there is the 

 same divergence of habits. There are ground-pigeons, rock- 

 pigeons, and wood-pigeons, — seed-eating pigeons and fruit- 

 eating pigeons ; there are carrion-eating, insect-eating, and 

 fruit-eating crows. Even kingfishers are, some aquatic, some 

 terrestrial in their habits ; some live on fish, some on insects, 

 some on reptiles. Lastly, among the primary divisions of birds 

 we find a purely terrestrial group — the Ratita?, including the 

 ostriches, cassowaries, etc. ; other great groui^s, including the 

 ducks, cormorants, gulls, penguins, etc., are aquatic ; while the 

 bulk of the Passerine birds are aerial and arboreal. The 

 same general facts can be detected in all other classes of 

 animals. In the mammalia, for example, we have in the common 

 rat a fish-eater and flesh-eater as well as a grain-eater, Avhich 

 has no doubt helped to give it the power of spreading over 

 the world and driving away the native rats of other countries. 

 Throughout the Kodent tribe we find everywhere aquatic, 

 terrestrial, and arboreal forms. In the weasel and cat tribes 

 some live more in trees, others on the ground ; squirrels have 

 diverged into terrestrial, arboreal, and flying species ; and 

 finally, in the bats we have a truly aerial, and in the whales 

 a truly ac{uatic order of mammals. We thus see that, 

 beginning with diff'erent varieties of the same species, we 

 have allied species, genera, families, and orders, with similarly 

 divergent habits, and adaptations to different modes of life, 

 indicating some general princi])le in nature which has been 

 operative in the development of the organic world. But in 

 order to be thus operative it must be a generally useful 

 principle, and Mr. Darmn has very clearly shown us in what 

 this utility consists. 



Divergence leads to a Maximum of Organic Forms in each Area. 



Divergence of character has a double purpose and use. In 

 the first place it enables a species which is being overcome 



1/ 



