286 RECAPITULATION. [Chap. XV. 



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 con- 

 stant 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 archi- 

 tectural 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 under- 

 stand 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 varie- 

 ties, we can at once see why their crossed offspring 

 should follow the same complex laws in their degrees 



