460 RECAPITULATION. 



if all possessed the same colored flowers ? If species ara 

 only well-marked varieties, of which the characters have 

 become in a high degree permanent, we can understand this 

 fact ; for they have already varied since they branched off 

 from a common progenitor in certain characters, by which 

 they have come to be specifically distinct from each other ; 

 therefore these same characters would be more likely again 

 to vary than the generic characters which have been in- 

 herited without change for an immense period. It is inex- 

 plicable on the theory of creation why a part developed in 

 a very unusual manner in one species alone of a genus, and 

 therefore, as we may naturally infer, of great importance to 

 that species, should be eminently liable to variation ; but, 

 on our view, this part has undergone, since the several 

 species branched off from a common progenitor, an unusual 

 amount of variability and modificLtion, 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 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 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 architectural 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 de- 

 scended from a common parent, and having inherited much 

 in common, we can understand how it is that allied species, 

 when placed under widely different conditions of Hfe, 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 

 ^election, we need not marvel at some instincts being not 



