2 4 o POPULAR SCIENCE MONTHLY. 



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, dis- 

 tinctively 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 special- 

 ized branches become the centers or stem forms of new radiations. 



The Mammals of Four Continents. 



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 Aus- 

 tralian region; from this extremely reptilian and egg-laying mono- 

 treme 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 radia- 

 tion in Australia and despite the disadvantage of starting from a spe- 

 cialized 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 mar- 

 supial and of the subsequent placental radiations; and our fuller 

 knowledge of them after twenty-five years of research is at once satis- 

 fying and disappointing, satisfying because it gives us prototypes of 

 the higher or modern mammals, disappointing because few if any of 



