Moreover, we have seen that the history of vertebrates is really the history of the development of the cerebrum, forebrain or large brain, as we call it in man. This is the seat in man of consciousness, thought, and will. This portion as a distinct and new lobe first appears in lowest vertebrates, increases steadily in size from class to class, reaches its most rapid development by mammals, and its culmination in man. During the tertiary period--the last of the great geological periods--the brain in many groups of mammals increased in size, both absolutely and relatively, eight to tenfold. Dr. Holmes says, that the education of a child should begin a century or two before its birth; man really began his mental education at least as early as the appearance of vertebrate life. But man is a mammal. This means that every organ is at its best. The digestive system, while making but a small part of the weight of the body, and built mainly on the old plan, is wonderfully perfect in its microscopic details. The muscles are heavy and powerful, arranged with the weight near the axis of the body, and replaced near the ends of the appendages by light, tough sinews. The higher mammal is this compact, light, and agile. The skeleton is strong, and the levers of the appendages are fitted to give rapidity of motion even at the expense of strength. And this again is possible only because of the high development and strength of the muscles. Moreover, the highest mammals are largely arboreal, and in connection with this habit have changed the foreleg into an arm and hand. The latter became the servant of the brain and gave the possibility of using tools. But increase in size and activity, and the expense of producing each new individual, led to the adoption of placental development. And the mammal is so complex, the road from the egg to the fully developed young is so long, that a long period of gestation is necessary. And even at birth the brain, especially of man, is anything but complete. Hence the necessity of the mammalian habit of suckling and caring for the young. And this feebleness and dependence of the young had begun far below man to draw out maternal tenderness and affection. And the mammalian mode of reproduction and care of young led to a more marked difference and interdependence between the sexes.