Innumerable modifications in accordance with altered use or disuse, such
as the enlarged udders of cows and goats, and the diminished lungs and
livers in highly bred animals that take little exercise, can be readily
and fully explained as depending on selection. As the fittest for the
natural or artificial requirements will be favoured, natural or
artificial selection may easily enlarge organs that are increasingly
used and economize in those that are less needed. I therefore see no
necessity whatever for calling in the aid of use-inheritance as Darwin
does, to account for enlarged udders, or diminished lungs, or the thick
arms and thin legs of canoe Indians, or the enlarged chests of
mountaineers, or the diminished eyes of moles, or the lost feet of
certain beetles, or the reduced wings of logger-headed ducks, or the
prehensile tails of monkeys, or the displaced eyes of soles, or the
altered number of teeth in plaice, or the increased fertility of
domesticated animals, or the shortened legs and snouts of pigs, or the
shortened intestines of tame rabbits, or the lengthened intestines of
domestic cats, &c.[44] Changed habits and the requisite change of
structure will usually be favoured by natural selection; for habit, as
Darwin says, "almost implies that some benefit great or small is thus
derived."