126 THE LIVING WORLD. 



(Toxodontia, Condylarthra, Amblypoda, Mesotheria, 

 Tillodontia) are confined to the Tertiary (9). The 

 duration of the existence of the above-mentioned 

 orders was short. In many cases the orders were 

 confined to one system of rocks. Of course the 

 sudden appearance of these various orders is, in a 

 measure, misleading. They always appear after a 



Q 



7> 

 Cr 



Tr 



P 



C 



D 



FIG. 1 6. 



long period of unrecorded history, commonly at the 

 beginning of a new system of rocks, and this fact 

 must be interpreted as meaning that their seemingly 

 sudden appearance is due to their earlier history 

 having been lost. In some of the orders mentioned, 

 indeed, the appearance was not very abrupt. But, 

 nevertheless, all of these groups of animals are 

 marked from others by their rapid development and 

 sudden extinction. 



