4:16 RECAPITULATION. [Chap. XV. 



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

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

 stance, should the colour of a flower be more likely to vary in 

 any one species of a genus, if the other species possess differently 

 coloured flowers, than if all possessed the same coloured flowers? 

 If species are only well-marked varieties, of which the characters 

 have become in a high degree permanent, we can understand thia 

 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 cha- 

 racters which have been inherited without change for an immense 

 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 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 

 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 

 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 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 

 slowly acquired through natural selection, we need not marvel 



