644 LAND MAMMALS IN THE WESTERN HEMISPHERE 



while not conclusive, is decidedly in favour of the latter view. 

 In the Triassic of South Africa is found a group of reptiles 

 which approximated the mammals very much more closely 

 than do any other known representatives of the lower verte- 

 brates. While it is not believed that any of these Triassic 

 reptiles were directly ancestral to the mammals, they did, 

 to a very great extent, bridge the gap between the two classes 

 and show us what the reptilian ancestors of the mammals were 

 probably like. 



With perhaps the exception of certain Insectivora, the Pal- 

 eocene faunas contained few, if any, ancestors of modern mam- 

 mals. These originated in some region which has not been 

 identified, but may be plausibly conjectured to be central 

 Asia, whence they migrated westward to Europe and eastward 

 to North America, reaching both of those continents in the 

 lower Eocene. From that time onward they increased and 

 multiplied, becoming more and more differentiated through 

 divergent evolution, until the existing state of things was at- 

 tained. From the lower Eocene we are on firm ground, and, 

 though very much remains to be learned, much has already 

 been accomplished in the way of tracing the history and develop- 

 ment of many mammalian orders. It has been my endeavour 

 in the body of this book to sketch the better established and 

 more significant parts of this marvellous story. 



