Chap. VII. SUMMARY. 243 



stincts vary slightly in a state of nature. No one will 

 dispute that instincts are of the highest importance to 

 each animal. Therefore I can see no difficulty, under 

 changing conditions of life, in natural selection accumu- 

 lating slight modifications of instinct to any extent, 

 in any useful direction. In some cases habit or use 

 and disuse have probably come into play. I do not 

 pretend that the facts given in this chapter strengthen 

 in any great degree my theory ; but none of the cases 

 of difficulty, to the best of my judgment, annihilate it. 

 On the other hand, the fact that instincts are not always 

 absolutely perfect and are liable to mistakes ; — that no 

 instinct has been produced for the exclusive good of 

 other animals, but that each animal takes advantage of 

 the instincts of others ; — that the canon in natural his- 

 tory, of " natura non facit saltum" is applicable to in- 

 stincts as well as to corporeal structure, and is plainly 

 explicable on the foregoing views, but is otherwise inex- 

 plicable, — all tend to corroborate the theory of natural 

 selection. 



This theory is, also, strengthened by some few other 

 facts in regard to instincts ; as by that common case of 

 closely allied, but certainly distinct, species, when in- 

 habiting distant parts of the world and living under 

 considerably different conditions of life, yet often retain- 

 ing nearly the same instincts. For instance, we can 

 understand on the principle of inheritance, how it is 

 that the thrush of South America lines its nest with 

 mud, in the same peculiar manner as does our British 

 thrush : how it is that the male wrens (Troglodytes) of 

 North America, build " cock-nests," to roost in, like the 

 males of our distinct Kitty- wrens, — a habit wholly unlike 

 that of any other known bird. Finally, it may not be 

 a logical deduction, but to my imagination it is far 

 more satisfactory to look at such instincts as the young 



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