the Obstetric Society of London,
on the second day of March, 1898, believe that we may trace here
a double menstruation, and would explain the phenomenon by
assuming that in certain cases there is an intermenstrual as well
as a menstrual cycle. The question is not yet ripe for
settlement, though it is fully evident that, looking broadly at
the phenomena of rut and menstruation, the main basis of their
increasing frequency as we rise toward civilized man is increase
of nutrition, heat and sunlight being factors of nutrition. When
dealing with civilized man, however, we are probably concerned
not merely with general nutrition, but with the nervous direction
of that nutrition.

At this stage it is natural to inquire what the corresponding phenomena
are among animals. Unfortunately, imperfect as is our comprehension of the
human phenomena, our knowledge of the corresponding phenomena among
animals is much more fragmentary and incomplete. Among most animals
menstruation does not exist, being replaced by what is known as heat, or
oestrus, which usually occurs once or twice a year, in spring and in
autumn, sometimes affecting the male as well as the female.[87] There is,
however, a great deal of progression in the upward march of the phenomena,
as we approach our own and allied zooelogical series. Heat in domesticated
cows usually occurs every three weeks. The female hippopotamus in the
Zooelogical Gardens has been observed to exhibit monthly sexual excitement,
with swelling and secretion from the vulva. Progression is not only toward
greater frequency with higher evolution or with increased domestication,
but there is also a change in the character of the flow. As Wiltshire,[88]
in his remarkable lectures on the "Comparative Physiology of
Menstruation," asserted as a law, the more highly evolved the animal, the
more sanguineous the catamenial flow.

It is not until we reach the monkeys that this character of the flow
becomes well marked. Monthly sanguineous discharges have been observed
among many monkeys. In the seventeenth century various observers in many
parts of the world--Bohnius, Peyer, Helbigius, Van der Wiel, and
others--noted menstruation in monkeys.[89] Buffon observed it among
various monkeys as well as in the orang-utan. J.G. St. Hilaire and Cuvier,
many years ago, declared that menstruation exists among a variety of
monkeys and lower apes. Rengger described a vaginal