296 ORIGIN OF SPECIES 



SUMMARY 



I have endeavored in this chapter briefly 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 importance 

 to each animal. Therefore there is no real difficulty, under 

 changing conditions of life, in natural selection accumulating 

 to any extent 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 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 can be shown to have been pro- 

 duced for the good of other animals, though animals take 

 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 inexplic- 

 able, — 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 distinct, species, when inhabiting distant parts of the 

 world and living under considerably different conditions of 

 life, yet often retaining nearly the same instincts. For in- 

 stance, 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 imprisoning 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 bird. Finally, it may not be 

 a logical deduction, but to my imagination it is far more satis- 



