218 ON THE ORIGIN OF SPECIES 



the structure or instincts of the fertile members, which 

 alone leave descendants. I am surprised that no one 

 has advanced this demonstrative case of neuter insects, 

 against the well-known doctrine of Lamarck. 



Summary. — I have endeavoured briefly in this chapter 

 to show that the mental qualities of our domestic 

 animals vary, and that the variations are inherited. 

 Still more briefly I have attempted to show that 

 instincts vary slightly in a state of nature. No one 

 will dispute that instincts are of the highest import- 

 ance to each animal. Therefore I can see no difficulty, 

 under changing conditions of life, in natural selection 

 accumulating 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 in- 

 stincts 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 history, of 'Natura non 

 facit saltum,' is applicable to instincts as well as to 

 corporeal structure, and is plainly explicable on the 

 foregoing views, but is otherwise inexplicable, — 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 re- 

 taining 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 



