192 Recent Geological Discoveries. 



This Australian animal therefore, remarkable for having, its false 

 "molars developed into broad grinding-teeth, different in form from 

 the true molars, is the representative of the oldest known type of 

 mammalian life. 



On the one hand, these discoveries show that many species of 

 mammals must have existed in the Oolitic period. On tlie other, 

 the predominance of Marsupial and Australian forms, along vf\\h 

 the great abundance and variety of contemporary reptilian life, 

 leads to the belief that the mammals occupied a subordinate 

 place, and that we are really approaching the time of their inti-o- 

 duction upon the stage. A question, however, arises here, and 

 is plainly stated by Sir Charles, which writers on this subject have 

 too often ignored. : How far is our knowledge of a general char- 

 acter ? and what do we know of the contemporary condition of 

 portions of the earth to which these discoveries do not apply ? 



"The advocates, however, of the doctrine of progressive de- 

 velopment will offer a different explanaton of the phenomena. 

 They will refer the large admixture of marsupials in the Stones- 

 field and Purbeck fauna to chronological lather than to climatal 

 conditions, — to the age of the planet rather than to the state of a 

 portion of its dry land. In the Triassic and Oolitic periods, they 

 will sa)^, the time had not yet come for the creation or develop- 

 ment of more highly oi'ganized beings. Experience muht test 

 and determine the soundness of these theoretical views. In the 

 meanwhile, it may be useful to bear in mind that while Australia 

 supports at present 100 species of marsupials, the rest of the con- 

 tinents and islands of the globe are tenanted by about ],Y00 spe- 

 cies of mammalia, of which only 4 6 are marsupials (namely, the 

 opossums of North and South America), and in like manner 

 there flourished in the Pliocene period throughout Europe, Asia, 

 and America, so far as we yet know, a placental fauna, consisting 

 of species now for the most part extinct, which was coeval with 

 the extinct Pliocene marsu|iSalsof Australia. Such facts, although 

 far too limited to enable us to. generalize Avith confidence, seem 

 rather to imply that at certain periods of the past, as in our own 

 days, the predominance" of certain families of terrestrial mamma- 

 lia, has had more to/do with conditions of space than of time, or 

 in other words, has been more governed by geographical circum- 

 stances than by a law of successive development of higher and 

 higher grades of organization, in proportion as the planet grew 

 older." 



