Man is, then, like the fishes, frogs, reptiles, birds, and quadrupeds, a vertebrate animal. Excepting in his infancy, when he is a quadruped going on all fours, he uses his posterior limbs only for locomotion, and his anterior for prehension and the like. His spinal axis is erect instead of horizontal, and his tail is atrophied. But he possesses all of the unmistakable qualities of the vertebrate type of structure—a two-chambered body cavity, a highly developed and dorsally located nerve trunk, vertebrate vitals, a closed circulatory system, a ventral heart, red blood, a head containing sense organs and brain, and a well-ordered internal skeleton, consisting of a vertebral column with skull and ribs and two pairs of limbs, the limbs consisting each of one long bone, two long bones, two transverse rows of irregular bones, and five branches at the end. 1. See ‘Classes of Animals,’ at the end of the chapter. 2. Snakes are limbless, and hind-limbs are lacking in whales and other degenerates; but rudimentary limbs are found in the embryonic stages of these animals. Frogs, it may be said also, have no ribs. III. Man a Mammal. Man is a _mammal_. He belongs to the most brilliant and influential of the five classes of vertebrates—the class to which belong so many of his associates and victims, the class to which belong the horse, the dog, the deer, the ox, the sheep, the swine, the squirrel, the camel, the unattenuated elephant, and the timid-hearted hare. To this class belong also the lion, the tiger, the kangaroo, the beaver, the bear, the bat, the monkey, the mole, the wolf, the ornithorhynchus, and the whale—in short, _all animals that have hair_. Fishes and reptiles have scales; birds have feathers; all mammals are covered to a greater or less extent with hair. The aquatic habits of whales render hair of no use to them. Hence, while the unborn of these animals still cling to the structural traditions of their ancestors and are covered with hair, the adults are almost hairless. The sartorial habits of human beings and the selective influences of the sexes have had a similar effect on the hairy covering of the human body. Hair exists all over the human body surface, excepting on the soles of the hands and feet, but in a greatly dwarfed condition. It is only on the scalp and on the faces of males, where it is scientifically assisted for purposes of display, that it grows luxuriantly. It is by no means certain that even the hair on the masculine scalp will last forever. For if the hermetical derby and other deadly devices worn by men continue their devastations as they have in the past, we may expect to have, in the course of generations, men with foreheads reaching regularly to the occiput. Most animals lay eggs. Man does not. Like the dog, the horse, the squirrel, and the bat, man is viviparous, the eggs hatching within the parental body. Human young are born helpless, and are sustained during the period of their infancy by the