NATURAL SELECTION 241 
DARWIN’S SUMMARY 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 
qualities 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 
