RECAPITULATION AND CONCLUSION 427 



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 

 sometimes 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 considerably different con- 

 ditions of life, yet should follow nearly the same 

 instincts ; why the thrush of South America, for 

 instance, lines her nest 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 apparently not perfect and 

 liable to mistakes, and at many instincts causing other 

 animals to suffer. 



If species be only well-marked and permanent varie- 

 ties, 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 

 acknowledged varieties. On the other hand, these 

 would be strange facts if species have been independ- 

 ently created, and varieties have been produced by 

 secondary laws. 



If we admit that the geological record is imperfect 

 in an extreme degree, then such facts as the record 

 gives, 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 



