ALLEN: NEW FOSSIL MAMMALS FROM CUBA. 11 



this hypothetical land mass into islands, whether by depression, by 

 the erosion of ocean currents, or by other geological processes, has 

 separated members of a once more homogeneous fauna, and through 

 long isolation they have in many cases developed racial variations 

 on the different islands. 



The time is not ripe for conclusions as to the place and method of 

 origin of the West Indian fauna. The evidence of fossil mammals is 

 still inconclusive. For while the numerous species and genera of 

 sloths and hystricine rodents recall strongly the characteristic South 

 American forms, the hystricines are of wide distribution in both hemi- 

 spheres, and insectivores are, so far as known, wholly absent from 

 South America until very recent times. Nevertheless the more 

 obvious view seems to be that the mammal fauna reached these areas 

 at a rather remote time, perhaps in part as more primitive types in a 

 retreat before a fauna of more specialized invaders from a northern 

 center of distribution, as argued so ably by Matthew (1915). A 

 severance of land connections with the continent would be then pos- 

 tulated, so that the ancient fauna might survive apart from further 

 competition with more modern forms. 



