474 
BECAPITULATIOK. 
Chap. XIV. 
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 enor¬ 
mous period. It is inexplicable on the theory of crea¬ 
tion 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 archi¬ 
tectural powers of the hive-bee. Habit no doubt some¬ 
times 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. 
