Masterpieces of Science 



this part has undergone, since the several species 

 branched off from a common progenitor, an un- 

 usual 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 sub- 

 ordinate 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 certain 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 in- 

 stincts; 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 

 undertand how it is that allied species, when 

 placed under widely different conditions of life, 

 yet follow nearly the same instincts; why the 

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