228 ANNALS NEW YORK ACADEMY OF SCIENCES 



islands. With bats, as with most birds, the intervening ocean acts as a 

 hindrance, but their wider distribution shows that it is less of a hindrance 

 than with terrestrial mammals. 



RODENTIA 



The abundant and dominant order of' Rodentia lends, in general, 

 strong support to the theories here advocated; but there are certain 

 serious difificulties which can be reconciled only by appealing to the im- 

 perfection of the geological record. 



The rabbits and picas form a group apart, the former Nearctic, the 

 latter Pal^arctic since the Oligocene, and both Holarctic since the 

 Pleistocene, the rabbits having extended their range over most of the 

 Oriental region and a large part of the Ethiopian and Neotropical. A 

 single specimen is recorded as from the Pleistocene of South America; 

 their introduction to Australia is known to have been by civilized man. 



Of the remaining rodents, the myomorph families are evidently of 

 Holarctic origin, as they first appear in Europe and N"orth America in 

 the Oligocene and the highest and most progressive modern types (e. g., 

 Arvicolinse) are now Holarctic, while in the southern continents they 

 are unknown until the Pleistocene and various primitive survivals are 

 found still living in Oriental, Ethiopian and Neotropical regions. We 

 may note, however, that the very abundant and typical group of Cricetinge 

 has its most primitive living representatives in tropical regions, that as 

 we go south in South America^ the genera approximate more toward the 

 more specialized arvicoline type, in the same way that they do as we go 

 northward in the northern continents.**' Since there is no doubt that 

 the cricetines are of northern origin, appearing first in South America 

 in the Pliocene or Pleistocene, while they are common in the Holarctic 

 regions from the Oligocene to the present day, we must suppose that the 

 higher development of the Antarctic genera, to which Oldfield Thomas 

 has called attention, is a case of parallelism with that of the Arctic 

 genera and that the colder climate of the far south is the stimulus which 

 reversed the usual conditions of geographical distribution. A review of 

 the fauna of the Argentine as compared with that of tropical South 

 America tends to show, I think, that this condition is general throughout, 

 and that the fauna is more progressive and more nearly equivalent in 

 development to those of the northern world than is that of the intervening 

 tropical zone. This is equally true of autochthonic races and of those 

 which are demonstrably of northern origin. Compare distribution of the 



*8 Oldfield Thomas. 



