232 SUMMARy. Chap. VII. 



we can understand, on the principle of inheritance, how it is 

 that the thrush of tropical South America hues its nest -with 

 mud, in the same peculiar manner as docs our British thrush : 

 how it is that the liornbills 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 throug-h Avhich the males feed them and the young 

 Avhen 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 lnrva3 of ichneumonidas feeding within the 

 live bodies of caterj:)illars — not as specially endowed or created 

 instincts, but as small consequences of one general laAV, lead- 

 ing to the advancement of all organic beings — namely, multi- 

 ply, vary, let the strongest live and the weakest die. 



