RECAPITULATION AND CONCLUSION 425 



species of the same genus differ from each other, be more variable 

 than generic characters in which they all agree? Why, for instance, 

 should the color of a flower be more likely to vary in any one 

 species of a genus, if the other species possess differently colored 

 flowers, than if all possessed the same colored flowers? If species 

 are only well-marked varieties, of which the characters have be- 

 come in a high degree permanent, we can imderstand this fact; 

 for they have already varied since they branched off from a com- 

 mon 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 inherited without change for an im- 

 mense period. It is inexplicable 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 impor- 

 tance 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 

 variabihty 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 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 in- 

 stincts. 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 in- 

 herited much in common, we can understand 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 



