我来打脸neo引用的这段好了：
Thus the weak members of civilized societies propagate their kind. No one
who has attended to the breeding of domestic animals will doubt that
this must behighly injurious to the race of man. It is surprising how
soon a want of care, orcare wrongly directed, leads to the degeneration
of a domestic race; but excepting in the case of man himself, hardly
any one is so ignorant as to allow his worst animals to breed.