582 PALEONTOLOGY 



were altogether no less than six orders of reptiles which had inde- 

 pendently abandoned terrestrial life and acquired more or less 

 perfect adaptation to aquatic life. Nature, limited in her resources 

 of outfitting for aquatic life, fashioned so many of these animals into 

 like form, it is small wonder that only within the last two years have 

 we finally distinguished all the similarities of analogous habit from 

 the similarities of real kinship. 



The most conservative members of this second branch are the 

 terrestrial, four-footed, persistently saurian or lizard-like forms, the 

 tuateras and the true lizards; but from these types again there radi- 

 ated off one of the marine orders (Mosasauria), 1 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. 2 



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 centres 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 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. 



1 For the origin of the Mosasaurs see L. Dollo, Les Ancetres des Mosasauriens, 

 Bulletin Scientifique de la France et dela Belgique, t. 38, pp. 137-139. 



F. Baron Nopcsa, Origin of the Mosasaurs, Geological Magazine, N. S. dec. iv, 

 vol. x, no. 465, March, 1903, pp. 119-121. 



S. W. Williston, The Relationships and Habits of the Mosasaurs, Journal of 

 Geology, vol. xn, no. 1, Jan.-Feb. 1904. 



2 See note 2, p. 581. 



