Among the reptiles the task of the mother ends, as a rule, with
the laying of the egg. One or two modern reptiles hatch the eggs,
or show some concern for them, but the characteristic of the
reptile is to discharge its eggs upon the warm earth and trouble
no further about its young. It is a reminiscence of the warm
primitive earth. The bird and mammal, born of the cooling of the
earth, exhibit the beginning of that link between mother and
offspring which will prove so important an element in the higher
and later life of the globe. The bird assists the development of
the eggs with the heat of her own body, and feeds the young. The
mammal develops the young within the body, and then feeds them at
the breast.
But there is a gradual advance in this process. The Duckbill lays
its eggs just like the reptile, but provides a warm nest for them
at the bottom of its burrow. The Anteater develops a temporary
pouch in its body, when it lays an egg, and hatches the egg in
it. The Marsupial retains the egg in its womb until the young is
advanced in development, then transfers the young to the pouch,
and forces milk into its mouth from its breasts.
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