ave 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
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
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 tropical and
temperate 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, 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