can 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 groups, 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, 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, we 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 by rivals, or is in
process of extinction by enemies, to save itself by adopting new habits
or by occupying vacant places in nature. This is the immediate and
obvious effect of all the numerous examples of divergence of character
which we have pointed out. But there is another and less obvious result,
which is, that the greater the diversity in the organisms inhabiting a
country or district the greater will be the total amou