MATTHEW, CLIMATE AND EVOLUTION 



229 



genera of Procyonidte, Canida, Cervidse, Tatuid^e and DasypodidiB among 

 mammals. 



Among the sciuromorphs, the squirrels are of early appearance (Oligo- 

 cene) in the northern world but are now most abundant in the East 

 Indies. The more specialized and later appearing marmots are chiefly 

 Holarctic. The highly specialized beavers and pocket-gopliers are Hol- 

 arctic and Nearctic respectively, from their first appearance. A marked 

 exception to the rule is seen in the survival in the western Sonoran sub- 



^ Qeomyidae. 

 ;sN^ He tero ?ni/i'dde 

 Pedeti'dde 

 Anomaluru 



Aynfestors of. 

 .(^eoini/ida'e lund 

 He !ero myidae 



Tertiary 

 ^Therldo mijtddi 

 ?Anomalu roideoj 



Fig. 13. — Distribution of Geomyoidea, Anomalitriclw and Pedetidce 



The Geomyoidea are of Nearctlc origin, but the more primitive Heteromyidse have 

 spread into part of South America. The Anomaluridae are thought to be the nearest 

 living relatives of the early Tertiary Theridomyidse. The Pedetidse are an aberrant 

 specialization, derived perhaps from the same group. 



region of Aplodontia, the most primitive living sciuromorph in several 

 respects. I have no explanation to offer of this anomaly, save that we 

 have not yet balanced properly the essential qualities of progressiveness 

 among Eodentia. 



Among the hystricomorphs, we find serious difficulties in the distribu- 

 tion. The most primitive living group is certainly the Anomaluridae 

 of West Africa; but, like the Pedetidae of South Africa, they offer a 



