EVIDENCE FROM DISTRIBUTION 125 



of distribution, it will be necessary to consider the 

 factors which determine distribution under present 

 conditions and, in order to keep the whole great 

 subject within manageable limits, we must confine 

 our attention to a few groups of organisms. For 

 our purposes by far the most suitable examples are 

 those furnished by the mammals. All the evidence 

 at our disposal goes to show that every species orig- 

 inated in some one particular area and has spread 

 from that starting-point as far as circumstances 

 would permit. This is called the "doctrine of 

 specific centres." In the case of more comprehen- 

 sive groups also, there is no reason to believe that, 

 among the higher animals, at least, the same genus, 

 family, order, class, etc., ever arose twice independ- 

 ently in disconnected land areas. This being the 

 case, the possession of identical, or closely similar, 

 animals by two regions now separated is strong evi- 

 dence that these regions were once joined and at 

 no very distant date. The spread of new forms from 

 their centres of origin may be quick or slow, as cir- 

 cumstances are or are not favourable; very rapid 

 spread was shown by the horses which, set free 

 about 1537, on the abandonment of the first settle- 

 ment of Buenos Aires, within forty-three years ap- 

 peared on the Straits of Magellan, nearly a thousand 

 miles away. 



The nature of the barriers which limit the spread 

 of any species differs with the kinds of organisms, 

 but for land mammals the most important are wide 



