234 Summary. Chap, viil 



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 fecit 

 sal turn," is applicable to instincts as well as to corporeal structure, 

 and is plainly explicable on the foregoing views, but is other- 

 wise 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 under- 

 stand, 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 larvas of ichneumonida? feeding within the 

 live bodies of caterpillars, — not as specially endowed or created 

 instincts, but as small consequences of one general law leading to 

 the advancement of all organic beings, — namely, multiply, vary* 

 let the strongest live and the weakest die. 



