176 MATTHEW— RECENT DISCOVERIES OF 



their small size they would not be crushed by the fall. The distri- 

 bution of the infra-mammalian groups in the West Indies, as on all 

 other oceanic islands, appears to me to be much more in accord with 

 this method of transportation than with any continental bridge 

 theories or current borne drift. Mammals, however, could hardly 

 be successfully colonized in this way, nor would it serve for chelo- 

 nians or alligators. But it does probably apply to most birds and 

 nearly all bats. 



The second feature in regard to the infra-mammalian groups is 

 that we know little or nothing of their past distribution. From such 

 slight direct evidence as we have, and from the analogies of past 

 distribution of the mammals, we can be reasonably sure that it was 

 not the same in the later Tertiary as it is now, and on the same evi- 

 dence I am confident that the methods of inferring past from pres- 

 ent distribution generally used by zoogeographers are erroneous. 

 If applied to mammals such methods would lead to conclusions ab- 

 surdly in conflict with the known facts ; they are in conflict with the 

 few available data among the lower land animals ; and they appear 

 to me to be based upon erroneous theoretical reasoning. 



Summarizing the data briefly we have among the mammals: 



1. The ground sloths, probably of South American origin (but 

 possibly via Central America) which must have arrived about the 

 late Miocene or early Pliocene, and demand isolation since that 

 date to account for their diversity, archaic type, and absence of later 

 developed and more progressive relatives. 



2. The rodents, all Hystricomorphs broadly of South American 

 affinities, but including three groups. 



(c) Chinchillids, limited to the eastern islands, and most diverse 

 and peculiar. They also have come from South America, but from 

 the eastward, in the late Miocene or Pliocene. 



(b) Hutias, including an eastern and a western group (but the 

 division line is not the same as with the chinchillids), the western 

 group with one near relative in Venezuela, the eastern genera (Hayti 

 and Porto Rico) more isolated. I have formulated several difter- 

 ent hypotheses to account for the distribution, but none can be 

 sufficiently tested by facts to be of much account. The clue lies, I 

 think, in a knowledge of the Pliocene and Pleistocene fauna of Cen- 



