144 THE EVOLUTION OF MAN. 



tlie young. Intimately connected with these, other im- 

 portant structural modifications gradually occurred. 



The period at which these important advances, whicli 

 laid the first foundation of the Mammalian class, took place, 

 may most probably be placed in the first part of the 

 Mesolithic, or Secondary Epoch, in the Triassic Period. 

 For the oldest known fossil remains of Mammals occur in 

 sedimentary rock -strata of the most recent deposits of the 

 Triassic Period, in the upper Keuper. It is possible, 

 indeed, that the parent-forms of Mammals may have 

 appeared earlier (perhaps even at the close of the Palseo- 

 lithic Epoch, in the Permian Period). But no fossil remains 

 of Mammals belonging to that period are as yet known. 

 Throughout the Mesolithic Epoch, throughout the Triassic, 

 Jurassic, and Calcareous Periods, fossil remains of Mammals 

 are very scarce, and indicate a very limited development 

 of the whole class. During this Mesolithic Epoch, Reptiles 

 play the chief part, and Mammals are of quite secondary 

 importance. It is, however, especially significant and 

 interesting, that all mammalian fossil remains of the 

 Mesozoic Epoch belong to the older and inferior division 

 of Pouclied Animals (Marsujncdia), a few probably even 

 to the yet older division of the Cloacal Animals {Mono- 

 trema). Among them, no traces of the third and most 

 highly developed division of the Mammals, the Placental 

 Animals, have as yet been found. The last, to which Man 

 belongs, are much more recent, and their fossil remains do 

 not occur till much later — in the succeeding CsBnolithic 

 Epoch ; in the Tertiary Period. This paloeontological fact 

 is very significant, because it harmonizes perfectly with 

 that order of the development of ^tammals which is un- 



