So much for the modes of sexual intercourse in all animals; but,
with regard to the same phenomenon, there are definite laws followed
as regards the season of the year and the age of the animal.

Animals in general seem naturally disposed to this intercourse
at about the same period of the year, and that is when winter is
changing into summer. And this is the season of spring, in which
almost all things that fly or walk or swim take to pairing. Some
animals pair and breed in autumn also and in winter, as is the case
with certain aquatic animals and certain birds. Man pairs and breeds
at all seasons, as is the case also with domesticated animals, owing
to the shelter and good feeding they enjoy: that is to say, with those
whose period of gestation is also comparatively brief, as the sow
and the bitch, and with those birds that breed frequently. Many
animals time the season of intercourse with a view to the right
nurture subsequently of their young. In the human species, the male is
more under sexual excitement in winter, and the female in summer.