NATURAL SELECTION 241 



Darwin's sumsiary of his answer to the third difficulty, that of accounting 



for the acquisition and modification of instincts 



through natural selection 



I have endeavored in this chapter briefly to show that the mental 

 quaUties 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 prob- 

 ably 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 

 produced 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 cor- 

 poreal 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 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 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 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 satisfactory to look 

 at such instincts as the young cuckoo ejecting its foster-brothers, 

 ants making slaves, the larvae of ichneumonidae feeding within the 

 live bodies of caterpillars, not as specially endowed or created 



