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 discharge in a species of cebus in Paraguay, while Raciborski observed in the Jardin des Plantes that the menstrual haemorrhage in guenons was so abundant that the floor of the cage was covered by it to a considerable extent; the same variety of monkey was observed at Surinam, by Hill, a surgeon in the Dutch army, who noted an abundant sanguineous flow occurring at every new moon, and lasting about three days, the animal at this time also showing signs of sexual excitement.[90]

The macaque and the baboon appear to be the non-human animals, in which menstruation has been most carefully observed.  In the former, besides the flow, Bland Sutton remarks that “all the naked or pale-colored parts of the body, such as the face, neck, and ischial regions, assume a lively pink color; in some cases, it is a vivid red."[91] The flow is slight, but the coloring lasts several days, and in warm weather the labia are much swollen.