EARLY IIISTORY OF TILE VERTEBRATA.
The vertebrates probably originated as aquatic o• marine animals partly amphibious. From this point they split into two great sections, one, the fishes, becoming entirely marine, the other be coming amphibious aml giving rise to the land vertebrates. These amphibious animals are most nearly represented among modern groups by the newts and salamanders, which with the frogs and toads constitute the order Amphibia. From the primitive Amphibia budded ofT three great groups o• 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 o• less the especial prov ince of the others, and also that. of the fishes. The greater diversity and complexity of the eon ditions of life on the land have favored the evolu tion there of the highest types of vertebrates. The strictly terrestrial and the aerial life have brought out the highest degree of mechanical specialization of parts: the arboreal life has been among the COnditiong favoring the highest development of intelligence, and the races stand ing highest in this respect are arboreal o• ex hibit indications of descent from arboreal an cestors.
During the geological age of invertebrates. the vertebrates were probably being first evolved from lower animals. as soft eel-like creatures, not unlike the amphioxus which is still found living 011 the Brazilian coast, without limbs and without bony skeleton, and leaving no traces of their existence hi the sediments. During the Silurian period, however, and perhaps as low down as the Ordovician, appeared certain marine vertebrates, somewhat allied to the fishes, but in ferior to them in important points of organiza tion, called Ostraeodermi (q.v.). These had no
interim/ bony skeleton, but were covered by bony plates over the head and sometimes over the body. Some had curious flipper-like appendages corresponding to the pectoral tins 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 ostracoderms continue to exist, and true fishes appear and become the dominant forms of life. They are far more primitive than most modern fishes, less completely adapted for marine life, and in most of them the internal skeleton is hot calcified and the fins are very imperfectly de veloped. The most remarkable among them is the Dinichthys, remotely allied to the modern lung fish, but of huge size and its head covered by massive bony plates. The ostrac•oderms 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 bony plates, 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 calcifying 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 pacity for higher evolution.