Page:Popular Science Monthly Volume 66.djvu/244

240 terrestrial, four-footed, persistently saurian or lizard-like forms, the tuateras and the true lizards; but from these types again there radiated off one of the marine orders (Mosasauria), the limbless snakes (Ophidia), while the lizards themselves have in recent times diverged almost to the point of true ordinal separation.

The most highly specialized members of this second branch are of course the flying pterosaurs, of whose ancestry we know nothing. Also in a grand division by themselves there evolved the dinosaurs, distinctively terrestrial, ambulatory, originally carnivorous, and probably more or less bipedal animals. Not far from the stem of the dinosaurs was also the source of the birds, also distinguished by bipedalism.

The working plan of creation becomes day by day more clear; it is, that each group, given time and space, will not only be fruitful and multiply, but will diversify in the search for every form of food by every possible method. Specialization in the long run proves fatal; the most specialized branches die out; the members of the least specialized branches become the centers or stem forms of new radiations.

So it is among the mammals, in which these principles find new and beautiful illustrations, although our knowledge of the early phases is fragmentary in the extreme. Our sole light on the first phase, in fact, is that obtained from the two surviving monotremes of the Australian region; from this extremely reptilian and egg-laying monotreme phase it appears, although opinion is divided on this point, that before the Jurassic period (i. e., already in the Trias) two branches were given off, the placental, from which sprang all the modernized mammals and the marsupial.

The marsupials appear to have passed through an arboreal or tree life condition, something similar to that seen in the modern opossum. The marsupials found their opportunity for unchecked adaptive radiation in Australia and despite the disadvantage of starting from a specialized arboreal type (Huxley, Dollo, Bensley), through the later Cretaceous and entire Tertiary a richly diversified fauna evolves, partly imitating the placentals and partly inventing entirely new and very peculiar forms of mammals, such as the kangaroo.

The oldest placental radiation which is fully known is that which was first perceived in Europe and fully recognized by the discovery in 1880 of the basal Eocene mammals of North America—it may be called the Cretaceous radiation. These mammals are distinctly antique, small-brained, clumsily built, diversified, imitative both of the marsupial and of the subsequent placental radiations; and our fuller knowledge of them after twenty-five years of research is at once satisfying and disappointing, satisfying because it gives us prototypes of the higher or modern mammals, disappointing because few if any of