V 
NATURAL SELECTION 
109 
other hand, seize their prey upon the ground, and the former 
feed largely on reptiles and ofl'al 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 Ratitae, including the 
ostriches, cassowaries, etc.; other great grouj 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 fiesh-eater as well as a grain-eater, which 
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 Rodent 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 aquatic order of mammals. We thus see that, 
beginning with different varieties of the same species, wc 
have allied species, genera, families, and orders, with similarly 
divergent habits, and adaptations to different modes of life, 
indicating some general principle 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. Darwin 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 
