individual animal's mechanism. But no animal that was merely
complete in itself would be long sanctioned by nature. For an animal
species to survive, there must evidently, also, be proper provision for
the production of young, and the preservation of the species as well
as of the individual. Hence in an animal's physiology and psychology
we meet with a vast amount of unselfish provision, and its structure
and happiness are more essentially dependent on the good of its kind
than on its narrow personal advantage. The mammalia probably owe
their present dominant position in the animal kingdom to the
exceptional sacrifices made by them for their young. Instead of
laying eggs and abandoning them before or soon after hatching, the
females retain the eggs within their bodies until the development of
the young is complete, and thereafter associate with them for the
purposes of nourishment, protection, and education. In the matter of
the tail, for instance, already noted, the individual rabbit incurs the
disadvantage of conspicuousness for the rear, in order to further the
safety of the young.