MODE OF EVOLUTION OF THE VERTEBRATA.
At a very early period in the history of vertebrates, the internal rod or backbone, at first cartilaginous, became ossified and segmented into vertelirce, and four appendages developed which became fins, legs, or wings, for progression in water, on land, or in the air, respectively. Along the upper side of the backbone lay the spinal cord, holding the cells and nerves which co ordinated the action of the different parts of the and the anterior end of this cord early specialized into a brain. The sense-organs and the month were grouped around the brain, and tile bony structures which developed to protect them and facilitate their action constituted the skull, which became the most characteristic and highly differentiated part of the animal. \lany vertebrates developed various protective bony structures over the surface of the body as well as the head.
The higher specialization of an animal, its more exact adaptation to a particular mode of existence, are an advantage to it so long as the conditions of its environment remain the same. But if the conditions change, the highly special ized type is at a disadvantage, as it cannot so well adapt itself to a new environment as its more generalized rivals. It is, therefore, likely to become extinct and be replaced by the less specialized forms, which in turn become highly specialized in adaptation to the new conditions. The many and great changes which have taken place many and complex changes in the conditions of life, both of marine and land animals, and changes in each group of organisms involve a widening circle of change in all the other groups dependent on it. But in the more complex modes of life a higher intelli gence and a better mechanical adaptation to the more general conditions of the environment have been at all times of advantage. We observe,
therefore, in the history of vertebrates through geological time, a general continuous progress in intelligence and in the broader lines of adapta tion, together with a series of successive speciali zations in the details of their construction.
The greater size of an animal is of advantage to it in the struggle for life as against its rivals or enemies, lint the larger animals require more food in proportion to their size in order to main tain the same relative activity. As long, there fore, as food is sufficiently abundant, a race of animals will tend to increase in size. But if changed conditions bring about a scarcity of food, the larger animals will become extinct and the smaller alone survive, and in their turn increase in size when a season of plenty recurs.
Accordingly we find that as a race of animals specializes in structure it likewise frequently in creases in size during stable conditions, and that with changed environment the larger and more specialized animals become extinct. But the improvement in the general mechanical adaptation of its parts enables an animal to at tain a greater size without corresponding loss in activity, and through geological time there has been a continual increase in the largest size attained by successive races of similar habits.
Besides struetures which serve to protect the animal or aid it in obtaining food or attack ing its enemies, the higher vertebrates develop ninny which are partly or wholly ornamental. The evolution of these is due to sexual selection and goes hand-in-hand with the development of extreme specializations and the increase in size which mark constant conditions of environment.