CHAP. XV.] RECAPITULATION. 391 



ance 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 

 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 being not perfect and liable to mis - 

 takes, and at many instincts causing other animals to suffer. 



If species be only well-marked and permanent varieties, we can 

 at once see why their crossed offspring should follow the same 

 complex laws in their degrees and kinds of resemblance to their 

 parents,^-in being absorbed into each other by successive crosses, 

 and in other such points, as do the crossed offspring of acknow- 

 ledged varieties. This similarity would be a strange fact, if 

 species had been independently created and varieties had been 

 produced through secondary laws. 



If we admit that the geological record is imperfect to an extreme 

 degree, then the facts, which the record does give, strongly support 

 the theory of descent with modification. New species have come 

 on the stage slowly and at successive intervals ; and the amount 

 of change, after equal intervals of time, is widely different in 

 different groups. The extinction of species and of whole groups 

 of species, which has played so conspicuous a part in the history 

 of the organic world, almost inevitably follows from the principle 



