1262 CLASS MAMMALIA. 



" whether the Monotremes are the descendants of the ancestral 

 Mammalia or not, it is quite certain that the higher Mammals must 

 at one time have passed through a condition such as now exists in 

 the Monotremes, in nearly all parts of their organisation ; and many 

 powerful arguments can be brought against the assumption that the 

 same stage has been reached independently, and at widely separated 

 periods, in the course of evolution." 1 



In the following chapters are given the leading palseontological 

 characters of each order of the class, with the range in time of the 

 different groups, and the names of the more important genera. The 

 number of genera is, however, so great that only a very brief and 

 general sketch of their characters and affinities can be given ; special 

 attention being drawn, where it may seem necessary, to those extinct 

 types which are of more than ordinary interest from an evolutionary 

 point of view. 



1 The opinion has been recently expressed by some Continental writers that the 

 Cetacea are the most archaic type of Mammals, and that they have been directly 

 derived from the Ichthyopterygian Reptiles. There is, however, so much evi- 

 dence against this view that it may be considered as practically disproved. 



