DISTRIBUTION OF SPECIES 



world are mostly located in the northern hemisphere, with the three prin- 

 cipal southern continents being more or less isolated, and centered in 

 regions which should have mild climates even in the more severe times. 



When severe climate begins in the polar regions and progresses south- 

 ward on the Holarctic land mass, the organisms which are there must 

 either adapt themselves to the new conditions, migrate southward, or be- 

 come extinct. As a result, the more progressive organisms, the ones which 

 succeed in adapting themselves to new conditions, should originate in 

 Holarctic centers, while the southern continents should become refuges of 

 primitive, less adaptable types. Matthew found that the paleontological 

 record supported this sequence of events for the mammals. All of the 

 major groups of mammals have arisen in the Holarctic, and the most 

 primitive mammals are and have been centered in the southern continents, 

 as the monotremes and marsupials in Australia. His study of the past and 

 present distribution of the mammals convinced him that extensive geo- 

 graphical changes are not necessary to explain the distribution of mam- 

 mals. That is, a sufficient explanation is contained in the permanence of 

 the continents with minor changes in level, alternating uniform warm and 

 zonal climates over the geological ages, and the use of the bridges de- 

 scribed above. Thus no appeal to undemonstrated phenomena such as 

 continental drift or former land bridges across the deepest of present 

 oceans is necessary. 



Matthew did not claim competence to judge the evidence from distri- 

 bution of invertebrates or plants, but he did believe that his theory would 

 have to be consistent with evidence from all groups in order to be valid. 

 He believed that this would prove to be true. 



Darlington's recent reappraisal of zoogeography reaffirms Matthew on 

 permanence of continents, but he finds much evidence that the major 

 groups of vertebrates arose in the Old World tropics, whence they invaded 

 the Holarctic and the rest of the world. All classes of vertebrates are most 

 abundant and varied in the tropics, and almost all of the dominant groups 

 are represented there, while few dominant groups are confined to the 

 north temperate zone. Much the same thing has been urged for plants for 

 many years by Camp and more recently by Axelrod. 



Related to this is Brown's idea of centrifugal speciation. As the largest 

 populations occur in the large, favorable areas, probability favors these 

 as places of origin of most of the progressive mutations. Migrations then 

 occur because of population pressure, and so the variability of a species 

 is spread over an increasing range. Should conditions become less favor- 

 able, the species will decline and retreat from much of its territory to the 

 most favorable parts. This leads to isolation and possibly to speciation. 

 Should some of the new species again be brought into contact, character 

 displacement should accelerate their further differentiation. The result is 

 a pattern of speciation from the center to the periphery of those large 

 areas which are most favorable for any particular group. 



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