Of course the great end of an animal is at first to fill its own belly, and in order to do this, if fixed as some molluscs are, it must contrive to bring nutriment within its reach, and if mobile limbs come to be developed to achieve locomotion, by fin in water, limb on land, and wing in air. After the vertebrate and mammal stage was achieved, the five-fingered limb takes various forms, as the paddle of the whale or wing of the bat. There are three great periods in geological development of animals--the Primary, which is, roughly speaking, the typical period of fishes; the Secondary, when reptiles prevail; and the Tertiary, the great age of mammals. Many geologists recognize a fourth period, the Post-Tertiary, Quaternary, or Diluvian, when existing species have been established. It is not till this latest period has arrived that we can detect unmistakable evidences of man. There are, however, many reasons which lead to the conclusion that his racial roots go still further back in time. Did he arise as a “mutation,” one of those rare sudden changes observed to take place even at the present time, by which a species suddenly departs from its ancestral type and is transformed? Let us briefly look at the main facts of mammalian ascent. The great herbivorous reptiles--some do not seem to have been strictly herbivorous--do not seem to lead us far on our path. Widely spread throughout the world, the Theriomorphs or beast-shaped reptiles seem to approach the mammal type, but they were too helpless and unwieldy, and had little brain-power wherewith to direct their energies. The earliest genuine mammals were small, not only relatively to those great creatures, but really little, rat-like rodents. Then we find arboreal creatures, driven to the trees for refuge and for food, squirrel-like animals, agile to escape from their monstrous but clumsy and stupid foes on the ground, and using their paws nimbly as hands to grasp and tear, or to break nuts and other food.