Physically he belongs to the great branch or phylum of vertebrate animals having a backbone--sometimes only a rod of cartilage--an internal locomotive skeleton, giving the possibility of great strength and swiftness, and of large size. Large size, with its greater heat-producing mass relative to its radiating surface, implies the possibility of warm blood, or constant high temperature, resulting in greater activity of all the organs, especially of the glands and the nervous system. Large size, as a rule, is accompanied by long life--giving opportunities for continuous and wide experience, and hence for intelligence. Yet most vertebrates have remained cold-blooded, and only a "saving remnant" even of men is really intelligent. Man belongs to the highest class of vertebrates, the Mammals, which produce living young and suckle them. Among the highest mammals, the Primates, or apes, the length of the periods of gestation, of suckling the young, and of childhood, with its dependence upon the mother, have become so long that she absolutely requires some sort of help and protection from the male parent. From this necessity have sprung various grades and forms of what we may venture to call family life, with all its advantages. How many mammals have attained genuine family life and how many men have realized its possibilities?[1] The upward march of our ancestors was neither easy nor rapid. They were anything but precocious. They were always ready to balk at progress, stiff-necked creatures who had to be driven and sternly held in the line of progress by stronger competitors. The ancestors of vertebrates maintained the swimming habit, which resulted in the development of the internal skeleton and finally of a backbone, not because it was easiest or most desirable, but because any who went to the rich feeding-grounds of the sea-bottom were eaten up by the mollusks and crabs. Our earliest air-breathing ancestors were crowded toward, and finally to the land, and into air-breathing by the pressure of stronger marine forms like sharks, or by climatic changes.[2] Reptiles, not mammals, dominated the earth throughout the Mesozoic era, and harried our ancestors into agility and wariness; at a later period the apes remained in the school of arboreal life mainly because the ground was forbidden and policed by the Carnivora. They and their forebears were compelled to forego some present ease and comfort, but always kept open the door to the future.