258 ANNALS NEW YORK ACADEMY OF SCIENCES 



partly to the prevalence of more uniform climatic conditions all over 

 the world and the consequent lack of environmental pressure tending to 

 force a change in habitat. Towards the end of the Eocene began a 

 period of progressively intensified elevation and disturbance, with re- 

 frigeration of climate beginning at the poles; this culminated in the 

 Glacial epoch. The northern fauna successively invaded the tropical 

 and southern continents and swept before it nearly all their autochthonic 

 faunae. 



In Africa, we see this invasion in progress in the Oligocene; the 

 anthracotheres, forerunners of the great ruminant invasion have already 

 appeared; to these may yet have to be added Palccomastodon as a fore- 

 runner of proboscidean invaders (although on the present record the 

 Proboscidea may appear an autochthonic group) ; while the hyracoids, 

 with Moerithcrium, Arsinoitherium, Barytherium and some less known 

 types are apparently autochthonic since Paleocene. Unfortunately, our 

 view stops here; we know little of the progress of this invasion until the 

 late Pliocene, when these invaders had themselves disappeared before a 

 succession of later invasions or become modified into new types. 



In South America, the isolation lasted much longer, and owing to the 

 great southward extension of the continent, a highly progressive inde- 

 pendent center of dispersal was set up in Argentina. Whatever criti- 

 cisms may be made of the phyletic theories of Dr. Ameghino, so far as 

 they affect the evolution of the mammalian races of the northern world, 

 I think that there can be no question that he has brought out a remark- 

 ably complete series of phyla in the autochthonic races of South America. 

 The closeness of these series, and the large amount of progressive evolu- 

 tion which they involve, on lines analogous to those of the northern 

 mammals, are fair indices that the controlling forces were similar and 

 that the southern end of the continent was the chief center of dispersal. 

 The various types of structure which were developed in northern mam- 

 mals during the Tertiary, in adaptation to the progressive change of 

 environment, are almost all paralleled, occasionally exceeded in degree 

 by these southern races; but they are very generally seen in different 

 combinations, as Professor Gaudry has so clearly shown.'^^ 



Had the Condylarthra reached Australia, we should expect to find 

 there a group of placental ungulate orders peculiar to the region, like 

 those of Tertiary South America, persisting to the present day. But we 

 find, instead, that the marsupials evolved into the herbivorous fauna. 

 In Madagascar the lemurs may be regarded as filling the place which 



" Albeet Gaddrt : Annales de Pal6ont., t. Ill, pp. 41-60. 1908. 



