Scrofula, rickets, and other diseases of bad conditions of life.
The udders and permanent milk of the domestic breeds of cow.
The 'fancy' rabbit's ears, drooping through lack of need to erect them. Dog's, ass's, etc., in some breeds ditto.
The obsolete eyes of mole and various cave-dwelling animals.
The diminished size of the wing-bones of domesticated ducks, due to ancestral disuse of flight.[570]
These are about all the facts which, by one author or another, have been invoked as evidence in favor of the 'lapsed intelligence' theory of the origin of instincts.
Mr. Darwin's theory is that of the natural selection of accidentally produced tendencies to action.
"It would," says he, "be the most serious error to suppose that the greater number of instincts have been acquired by habit in one generation, and then transmitted by inheritance in succeeding generations. It can clearly be shown that the most wonderful instincts with which we are acquainted, namely, those of the hive-bee and of many ants, could not possibly have been thus acquired.[571] It will be universally admitted that instincts are as important as corporeal structure for the welfare of each species, under its present conditions of life. Under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and continually accumulating variations of instinct to any extent that may be profitable. It is thus, as I believe, that all the most complex and wonderful instincts have arisen.... I believe that the effects of habit are of quite subordinate importance to the effects of the natural selection of what may be called accidental variations of instincts;—that is, of variations produced by the same unknown causes which produce slight deviations of bodily structure."[572]