2o8 THE CHAIN OF LIFE. 



are not found until we reach the beginning of the Tertiary. 

 The Monotremes would seem to be a comparatively modern 

 degraded type. Thus the Marsupials existed throughout the 

 reptilian age, and this in those countries of the northern 

 hemisphere in which they are not now found. The Mesozoic 

 Marsupials were, it is true, of small size, but there were 

 probably numerous species, and though unable to cope with 

 the great reptiles that swarmed by the shores and on the 

 plains, they may have found abundant scope in the upland 

 and interior regions of the continents. 



The Upper Trias of Germany has afforded to Professor 

 Pleininger two teeth of a small mammal, to which the name of 

 Aficrolestes antiquus has been given, under the impression that 

 it was carnivorous, though it now seems more likely that it 

 was a vegetable feeder. In rocks of nearly the same age in 

 America, Emmons found a jaw-bone of another species {Dro- 

 viatherium sylvestre), which appears to be a near ally of the ex- 

 isting Myrmecobius fasciatus of Australia (Figs. i66, 167). In 

 the Stonesfield slate, a member of the English Jurassic, several 

 othrr species have been found (Fig. 168), and a still larger 

 number in the freshwater beds of the Upper Purbeck. None 

 appear to have yet been found in the Cretaceous, but they re- 

 appear in the Eocene Tertiary, and continue to the modern 

 time. Their absence in the Cretaceous is probably a mere 

 accident, and they present an illustration of a very permanent 

 type little changed since its first introduction. Lyell enume- 

 rates in all thirty-three species from the Mesozoic, all of them 

 of small size, and all more or less nearly related to existing 

 Australian Marsupials, though differing much among them- 

 selves, and including both carnivorous and herbivorous forms 

 (Fig. 169). 



So soon as the palaeonlotogist passes from the Upper 

 Cretaceous to the Eocene, he finds himself in the domain of 

 the placental mammals, which appear in numerous and large 

 species, and this, not merely in one region, but in every part 



