276 SUMMARY. 



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 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 distinct, species, when inhabiting distant parts 

 of the world and living under considerable different con- 

 ditions of life, yet often retaining nearly the same instincts. 

 For instance, we can understand, on the principle of in- 

 heritance, how it is that the thrash of tropical South Amer- 

 ica 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 i^laster 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 ichneumonidse 

 feeding within the live bodies of caterpillars, not as spe- 

 cially endowed or created instincts, but as small conse- 

 quences of one general law leading to the advancement of 

 all organic beings — namely, multiply, vary, let the strong- 

 est live and the weakest die. 



