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 under- 

 stand 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 ease of neuter in- 

 sects, 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 tem- 

 perate 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 and kinds 



