As we pass higher up the animal scale, we find
the number of eggs or young more and more reduced, and the
diminution of their number compensated for by parental protection.
At the lowest stage this protection may consist in the provision of
some merely physical shelter, as in the case of those animals that
carry their eggs attached in some way to their bodies. But, except
at this lowest stage, the protection afforded to the young always
involves some instinctive adaptation of the parent's behaviour. We
may see this even among the fishes, some of which deposit their
eggs in rude nests and watch over them, driving away creatures that
might prey upon them. From this stage onwards protection of
offspring becomes increasingly psychical in character, involves
more profound modification of the parent's behaviour, and a more
prolonged period of more effective guardianship. The highest stage
is reached by those species in which each female produces at a
birth but one or two young, and protects them so efficiently that
most of the young born reach maturity; the maintenance of the
species thus becomes in the main the work of the parental instinct.
In such species the protection and cherishing of the young is the
constant and all-absorbing occupation of the mother, to which she
devotes all her energies, and in the course of which she will at
any time undergo privation, pain, and death.
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