branched off from a common progenitor
in certain characters, by which they have come to be specifically distinct
from each other; {474} and therefore these same characters would be more
likely still to be variable than the generic characters which have been
inherited without change for an enormous period. It is inexplicable on the
theory of creation why a part developed in a very unusual manner in any one
species of a genus, and therefore, as we may naturally infer, of great
importance to the species, should be eminently liable to variation; but, on
my 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 this 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 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, {475} 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 c