364 SUMMARY. [Chap. VIIl. 



tent slight modifications of instinct which are in any 

 way useful. In many 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 de- 

 gree 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 can 

 be shown to have been produced for the good of other 

 animals, though animals take advantage of the instincts 

 of others; — that the canon in natural history, of " Na- 

 tura non faeit 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 selec- 

 tion. 



This theory is also strengthened by some few other 

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

 closely allied, but distinct species, when inhabiting dis- 

 tant parts of the world and living under considerably 

 different conditions of life, yet often retaining nearly 

 the same instincts. For instance, we can understand, 

 on the principle of inheritance, how it is that the thrush 

 of tropical South America lines its nest with mud, in 

 the same peculiar manner as does our British thrush; 

 how it is that the Hornbills of Africa and India have the 

 same extraordinary instinct of plastering up and im- 

 prisoning the females in a hole in a tree, with only a 

 small hole left in the plaster through which the males 

 feed them and their young when hatched; how it is that 

 the male wrens (Troglodytes) of North America build 

 " cock-nests," to roost in, like the males of our Kitty- 

 wrens, — a habit wholly unlike that of any other known 



