304 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



temporary isolation of the Seychelles and the Indian 

 peninsula. 



We lack the exact palseontological data for 

 fixing the date of the separation of Africa from 

 South America, but, judging from the distribution 

 of marine deposits, it may very well be that the 

 connection of these two countries lasted down to 

 the time of the great Senonian incursion. 



III. MIGRATIONS IN TERTIARY TIMES. The study 

 of migrations in the Tertiary epoch becomes even 

 more interesting and more capable of precision 

 than those of Secondary times, thanks to the re- 

 markable development of the class of Mammals. 



The first appearance of the Mammals, so far as 

 our present knowledge goes, is indeed fixed at a 

 fairly remote date in the Secondary era. From 

 the higher Trias and the commencement of the 

 Rhsetian, there suddenly appeared two distinct 

 types of this class : on the one hand the Insecti- 

 vorous Marsupials, akin to the present Didelphidse 

 of the two Americas, appear for the first time in the 

 upper Trias of North Carolina ; on the other part 

 the Allotherians, or Multituberculata, as they are 

 called from the manifold series of tubercules which 

 bristle on the crowns of their molars, make their 

 appearance, in the diminutive forms of the family of 

 Plagiaulacidse, in the Rhsetian of Stuttgart and of 

 the South of England. This family perhaps still 

 exists at the present day as the Kangaroo-rats of 

 Australia. Were we to rely exclusively on known 

 facts, we should presume that the first centre of 

 dispersion of the Marsupials was North America, 



