[21] In the domesticated state, the sow is often permitted to have two
and even three litters in a year. This custom is very pernicious; it
debilitates the mother, overworks all parts of the living machinery, and
being in direct opposition to the laws of their being, their progeny
must degenerate. Then, again, let the reader take into consideration the
fact that members of the same litter impregnate each other, in the same
ratio, and he cannot but come to a conclusion that we have long since
arrived at--that these practices are among the chief causes of
deterioration.