490 RBOAPITULATION. 



become in a high degvee 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 inherited without change for an immense period. 

 It is inexplicable on the theory of creation why a part de- 

 veloped 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 pro- 

 genitor, 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 com- 

 mon to many subordinate forms, that is, if it has been in- 

 herited for a very long period; for in this case it will have 

 been rendered constant by long-continued natural selection. 

 Glancing at instincts, marvelous 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 inherited 

 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 at some instincts 



