16 LAND MAMMALS IN THE WESTERN HEMISPHERE 



It must not be supposed that all the divisions of similar 

 rank, such as the eras, for example, were of equal length, as 

 measured by the thickness of the rocks assigned to those divi- 

 sions. On the contrary, they must have been of very unequal 

 length and are of very different divisibility. The Pre-Cambrian 

 eras, with only two periods, were probably far longer than 

 all subsequent time, and all that the major divisions imply 

 is that they represent changes in the system of life of 

 approximately equivalent importance. It is impossible to 

 give any trustworthy estimate of the actual lengths of 

 these divisions in years, though many attempts to do so 

 have been made. All that can be confidently aflBrmed is 

 that geological time, like astronomical distances, is of in- 

 conceivable vastness and its years can be counted only in 

 hundreds of millions. 



To discuss in any intelligible manner the history of mammals, 

 it will be necessary to go much farther than the above table 

 in the subdivision of that part of geological time in which 

 mammalian evolution ran its course. As mammals repre- 

 sent the highest stage of development yet attained in the 

 animal world, it is only the lattor part of the earth's history 

 which is concerned with them; the earlier and incomparably 

 longer portion of that history may be passed over. Mammals 

 are first recorded in the later Triassic, the first of the three 

 periods which make up the Mesozoic era. They have also 

 been found, though very scantily, in the other Mesozoic periods, 

 the Jurassic and Cretaceous, but it was the Cenozoic era that 

 witnessed most of the amazing course of mammalian develop- 

 ment and diversification, and hence the relatively minute sub- 

 divisions necessary for the understanding of this history deal 

 only with the Cenozoic, the latest of the great eras. 



In the subjoined table the periods and epochs are those 

 which are in general use throughout the world, the ages and 

 stages are those which apply to the western interior of North 

 America, each region, even of the same continent, requiring a 



