474 KECAPITULATION. 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 dif- 

 ferent animals of the same class with their several in- 

 stincts. 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, 



