MATTHEW, CLIMATE AND EVOLUTION 271 



ently from a common center of dispersal. The relations are like those of 

 one side and the other of a branching tree whose trunk region is unknown 



to us. 



The more ancient and primitive groups of the Mammalia have mostly 

 disappeared, or are in process of disappearance, from Holarctica. In the 

 peripheral continents, they have undergone in many cases a notable local 

 adaptive radiation and expansion, extensive in proportion to the isolation 

 of these continents from the northern realm, more complete during the 

 early and middle Tertiary than now. When the reunion to Holarctica 

 permitted the northern faima to invade the peripheral continents, these 

 autochthonous groups were in general unable to maintain themselves 

 against the competition of the more progressive northern races, and have 

 either wholly disappeared or left a few scattered survivors, mostly aber- 

 rant specializations which did not come directly into competition with the 

 invading races. The survival of the major part of the marsupial radia- 

 tion in Australia is attributable to its continued isolation. The apparent 

 fact that Neotropical races of Edentata were able to invade North Amer- 

 ica during the Pliocene and Pleistocene may be ascribed to two factors : 



1) No Nearctic groups of closely analogous specialization existed at 

 that time. 



2) Owing to the far southerly extension of South America, the evolu- 

 tion of mammals in that region was, so far as controlled by climatic 

 change, more progressive and more nearly equivalent to the Holarctic 

 evolution than in Australia or Africa. Its products therefore were better 

 able to maintain themselves against their northern competitors. 



If we regard the Proboscidea as of Ethiopian origin, we must suppose 

 that they too constitute an exception to the general rule that the races 

 evolved in the peripheral regions have been unable to invade Holarctica. 

 But the recent discoveries of Pilgrim and Cooper in the Oligocene of 

 India tend strongly to show that the Proboscidea were from the first, as 

 they certainly were in the later Tertiary, a group of Asiatic, not African, 

 dispersal. 



The dominant influence of climate in controlling the range of modern 

 mammals has been emphasized by C. H. Merriam. The mammals adapted 

 to north temperate or even boreal climate are the most specialized and 

 last evolved members of their respective races. The most primitive sur- 

 vivors of northern races, and surviving members of races formerly abun- 

 dant in the north, are met with chiefly in tropical regions. Similar rela- 

 tions are seen in the f aimse of the antarctic as compared with the southern 

 tropical regions, although less obvious. This is especially seen in South 



