we shall presently have to deal, few attempts have been made to demonstrate any annual organic sexual rhythm.  The supposition of such annual cycle is usually little more than a deduction from the existence of the well-marked seasonal sexual rhythm in animals.  Most of the higher animals breed only once or twice a year, and at such a period that the young are born when food is most plentiful.  At other periods the female is incapable of breeding, and without sexual desires, while the male is either in the same condition or in a condition of latent sexuality.  Under the influence of domestication, animals tend to lose the strict periodicity of the wild condition, and become apt for breeding at more frequent intervals.  Thus among dogs in the wild state the bitch only experiences heat once a year, in the spring.  Among domesticated dogs, there is not only the spring period of heat, early in the year, but also an autumn period, about six months later; the primitive period, however, remains the most important one, and the best litters of pups are said to be produced in the spring.  The mare is in season in spring and summer; sheep take the ram in autumn.[128] Many of the menstruating monkeys also, whether or not sexual desire is present throughout the year, only conceive in spring and in autumn.  Almost any time of the year may be an animal’s pairing season, this season being apparently in part determined by the economic conditions which will prevail at birth.  While it is essential that animals should be born during the season of greatest abundance, it is equally essential that pairing, which involves great expenditure of energy, should also take place at a season of maximum physical vigor.

As an example of the sexual history of an animal through the year, I may quote the following description, by Dr. A.W.  Johnstone, of the habits of the American deer:  “Our common American deer, in winter-time, is half-starved for lack of vegetation in the woods; the low temperature, snow, and ice, make his conditions of life harder for lack of the proper amount of food, whereby he becomes an easier prey to carnivorous animals.  He has difficulty even in preserving life.  In spring he sheds his winter coat, and is provided with a suit of lighter hair, and while this is going on the male grows antlers for defence.  The female about this time is far along in pregnancy, and when the antlers are fully grown she drops the fawn.  When the fawns are dropped vegetation is plentiful and lactation sets in.  During this time the male is kept fully employed in getting food and guarding his more or less helpless family.  As the season advances the vegetation increases and the fawn begins to eat grass.  When the summer heat commences the little streams begin to dry up, and the animal once more has difficulty in supporting life because of the enervating heat, the effect of drought on the vegetation, and the distance which has to be traveled to get water; therefore,