eties, of which the characters have become in a high degree
permanent, we can understand this fact; for they have already varied
since they branched off from a common progenitor in certain characters,
by which they have come to be specifically different from each other;
therefore these same characters would be more likely again to vary than
the generic characters which have been inherited without change for an
immense period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in one species alone of a genus, and
therefore, as we may naturally infer, of great importance to that
species, should be eminently liable to variation; but, on our view,
this part has undergone, since the several species branched off from a
common progenitor, an unusual amount of variability and modification,
and therefore we might expect the part generally to be still variable.
But a part may be developed in the most unusual manner, like the wing of
a bat, and yet not be more variable than any other structure, if the
part be common to many subordinate 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 do corporeal structures 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 certain
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 often
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 widely different conditions of life,
yet follow nearly the same instincts; why the thrushes of temperate and
tropical South America, for instance, line their nests 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
not perfect and liable to mistakes,