Origin of Species Variorum

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 cannon 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 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 larvæ of ichneumonidæ 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.