ordinate forms, that 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 so conspicuous
a part in the history of the organic world, almost inevitably follows on
the principle of natural selecti