Another reason why the eggs of mammals are retained within the bodies of the females is the need for protecting the young animals from enemies. In the animal kingdom each kind of animal preys upon another kind. They attack and devour each other and are constantly in danger. If, then, mammals laid eggs in nests and sat upon them to hatch them out, the mother animals sitting on the nests would be continually in danger of attack from their enemies. They would either have to flee and subject the nest and its contents to the danger of destruction or else stay and fight, and perhaps be destroyed. But by carrying her egg within her body the mother mammal is able to move about from place to place and protect her baby. Is Man an Animal? Men, women and children belong to the “mammal” class of animals. The offspring of the human family is the most helpless of all animals at birth. The young of most kinds of mammals can stand on their legs shortly after being born, but the human baby requires months before it can stand up. A baby horse can also walk within a few hours, but human children do not begin to walk until they are more than a year old. Why Cannot Babies Walk as Soon as Born? The human baby has a great many more things to learn than a horse baby before it is safe for him to go about alone. It takes time for the brain to develop, and if a baby could walk before the brain had even partially developed it would only get into trouble. This, then, is what we have learned about the reproduction of life and the reasons for its being different in different classes of life. First, we had the division of organic life into the vegetable and animal kingdoms. Life in the vegetable kingdom has none of the five senses, for plants cannot see, hear, feel, smell or taste. They cannot move from place to place, but remain where they grow until destroyed or removed. On the other hand, all animal life has at least one of the five senses--feeling. The oysters and clams belong to this class. Starting with this level of life in the animal kingdom we find that as we go on up through the different classes we find each class able to do things which make it superior to the class below it, until we reach the human mammal, who can do most of all. And, further, that since each class as we go up in the scale of life has greater ability to do things than the class beneath it, so in each case the task of the parents in preparing their offspring for their kind of life becomes greater, and the period during which the offspring is learning becomes longer and longer until we reach the human family, in which we find that parents have the greatest responsibility, and the children are the most helpless of all animals, but that in the final result man has a right, on account of his superior qualities, to be the ruler of the other creatures of the world. What Are Ball Bearings?