VERTEBRATA. 98 it in the struggle for life as against its rivals or enemies, but the larger animals require more food in proportion to their size in order to main- tain the same relative activit}'. 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 'structures which serve to protect the animal or aid it in obtaining food or attack- ing its enemies, the higher vertebrates develop many 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. EARLY HISTORY OF THE VERTEBRATA. The vertebrat<'S probably originated as aquatic or marine animals partly amphibious. From this point they split into two great sections, one, the fishes, becoming entirely marine, the other be- coming amphibious and giving rise to the land vertebrates. These amphibious animals are most nearly represented among niodern groups by the newts and salamanders, which with the frogs aVid toads constitute the order Ampliibia. From the primitive Amphibia budded off three great groups or classes, the reptiles, predominantly amphib- ious, the mammals, predominantly terrestrial, and the birds, predominantly aerial. Each of those classes has since branched out into every available mode of life in its own province, and each has invaded more or less the especial prov- ince of the others, and also that of the fishes. The greater diversity and complexity of the con- ditions of life on the land have favored the evolu- tion there of the highest tyjies of vertebrates. The strictly terrestrial and the aerial life have brought out the highest degree of mechanical specialization of parts; the arlioreal life has been among the conditions favoring the highest development of intelligence, and the races stand- ing highest in this respect are arboreal or ex- hibit indications of descent from arboreal an- cestors. During the geological age of invertebrates, the vertebi'ates were probably being first evolved from lower animals, as soft eel-like creatures, not unlike the amphioxus which is still found living on the Brazilian coast, without limbs and without bony skeleton, and leaving no traces of their existence in the sediments. During the Silurian period, however, and perhaps as low down as the Ordovician, appeared certain marine vertebrates, somewhat allied to the fishes, Imt in- ferior to them in important points of orgnniza- tinn, called Ostracndermi (q.v.). These had no' internal bony skeleton, but were covered by bony VERTEBRATA. plates over the head and sometimes over the body. Some had curious llipper-like appendages corresponding to the pectoral fins of fishes; there was no separate lower jaw. They are so unlike any living animal that it is difficult to get any complete knowledge of their anatomy from the ex- ternal hard parts, which are all that is pre- served, for the internal skeleton was not calcified. In the succeeding or Devonian period the ostraeoderms continue to exist, and true fishes appear and become the dominant forms of life. They are far luore primitive than most modem fishes, less completely adapted for marine life, and in most of them the internal skeleton is iiot calcified and the fins are very imperfectly de- veloped. The most remarkable among them is the Dinichthys, remotel.v allied to the modern lung- fish, but of huge size and its head covered by massive bon,v plates. The ostraeoderms die out at the end of this period. Fishes appear in all the succeeding marine formations and become more and more like those of the present day. The next two periods, the Carboniferous and Permian, constitute the age of amphibians, when large and small Amphibia, mostly covered com- pletely with bonv jjlates, were the dominant forms of life. These animals were amphibious, some probably terrestrial, and had developed limbs and feet for progression on land. They show various stages in the calcif.ving of the backbone, and in some it is complete. In the Permian the first reptiles appear, primitive types not easily dis- tinguished from the Amphibia from which they had originated, but superior to them in their ca- paeit}' for higher evolution. Evolution of the Reptilia. During the Trias- sic, Jurassic, and Cretaceous periods the reptiles were the dominant types, and branched out into terrestrial, ai-rial. amphibious, and marine groups in great variety, and many of them, such as the dinosaurs (q.v.), were of gigantic size. With them lived ancestors of the modern crocodiles and turtles. The marine reptiles, ichthyosaurs, mosa- saurs, and plesiosaurs, were dominant on the ocean, while the pterosaurs or flying i-eptiles ruled the air. At the end of the Cretaceous most of the reptile orders became extinct. The first birds appear in the .Jurassic, retain- ing many characters of their reptilian ancestors, especiall.v reptile-like teeth and a long verte- brated tail, from which the tail feathers branch out on each side down to the tip. In the Cre- taceous period the birds have lost the reptilian tail, but still retain the teeth. All later birds have horny beaks instead of teeth. Evolution of the IMammalia. During the age of reptiles the mammals had been slowly evolving, but vi>rv little is known of them. The remains that have been found arc exceedingly rare and verv fragmentary, and indicate very primitive types, of minute size and apparently arboreal habit. But at the beginning of the Tertiary period they appear in force, and rapidl.v increase in size and variety, taking the place of the reptiles as (he dominant t,vpe, and like them branching out into terrestrial, arboreal, aerial, marine, and some amphibious races. The ances- try of many modern animals has been traced back through successive stages nearl.y to the beginning of the Tertiary period, always converging to- ward a common ancestor. The most complete and instructive of these lines of descent is that of the
