is, if it has been inherited for a very long period; for in this case
it will have been rendered constant by long-continued natural
selection.
Glancing at instincts, marvellous as some are, they offer no greater
difficulty than does corporeal structure on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation throws
on the admirable architectural powers of the hive-bee. Habit no doubt
sometimes comes into play in modifying instincts; but it certainly is
not indispensable, as we see, in the case of neuter insects, which
leave no progeny to inherit the effects of long-continued habit. On the
view of all the species of the same genus having descended from a
common parent, and having inherited much in common, we can understand
how it is that allied species, when placed under considerably different
conditions of life,
yet should follow nearly the same instincts; why the thrush of South
America, for instance, lines her nest with mud like our British
species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently
not perfect and liable to mistakes, and at many instincts causing other
animals to suffer.
If species be only well-marked and permanent varieties, we can at once
see why their crossed offspring should follow the same complex laws in
their degrees and kinds of resemblance to their parents, in being
absorbed into each other by successive crosses, and in other such
points, as do the crossed offspring of acknowledged varieties. On the
other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary
laws.
If we admit that the geological record is imperfect in an extreme
degree, then such facts as the record gives, support the theory of
descent with modification. New species have come on the stage slowly
and at successive intervals; and the amount of change, after equal
intervals of time, is widely different in different groups. The
extinction of species and of whole groups of species, which has played
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