Page:Creation by Evolution (1928).djvu/341

as we pass from the oldest amphibians to the oldest reptiles; thence to the theromorph series of reptiles, which became more and more mammal-like; thence to the oldest mammals, onward to the most primitive placental mammals of the closing age of the dinosaurs; thence to the tree-shrews, lemuroids, monkeys and apes that grow steadily more like man; finally to man himself in the last few million years of the billion-year history of life.

In their individual development or embryology the amphibians, like the fishes, went through a water-living stage of development, in which they had functional gills; but the earliest reptiles succeeded in laying their eggs and rearing their young wholly upon land and in this way were able not only to invade the drier uplands and many parts of the earth where water was scarce, but to avoid the intensive competitive warfare for living food that must have raged in the swamps of the coal forests.

The earliest reptiles were still so much like their contemporary relatives, the amphibians, in most of their skeletal characters that some of them were on or relatively near the borderland between the two classes. Such a form is Seymouria, from the Permian of Texas, a fine skeleton of which is mounted in the University of Chicago. The pattern of its skull bones, as seen from above and from the side, conforms closely to the primitive amphibian type and is an almost ideal archetype of every later skull, including that of mammals and of man himself. The same is true of the underside of the skull, including the arrangement of the numerous elements of the upper jaws, palate, and base of the cranium. It may be said of Seymouria, as of many other generalized forms, that, if we had not discovered them, we could have predicted their existence, so closely do they