346 GEOGRAPHICAL DISTRIBUTION. Chap. XI. 



CHAPTEE XI. 



Geographical Distribution. 



Present distribution cannot be accounted for by differences in phy- 

 sical conditions — Importance of barriers — Affinity of the pro- 

 ductions of the same continent — Centres of creation — Means 

 of dispersal, by changes of climate and of the level of the land, 

 and by occasional means — Dispersal during the Glacial period 

 co-extensive with the world. 



In considering the distribution of organic beings over 

 the face of the globe, the first great fact which strikes 

 us is, that neither the similarity nor the dissimilarity 

 of the inhabitants of various regions can be accounted 

 for by their climatal and other physical conditions. Of 

 late, almost every author who has studied the subject 

 has come to this conclusion. The case of America 

 alone would almost suffice to prove its truth : for if we 

 exclude the northern parts where the circumpolar land 

 is almost continuous, all authors agree that one of the 

 most fundamental divisions in geographical distribution 

 is that between the New and Old Worlds ; yet if we 

 travel over the vast American continent, from the 

 central parts of the United States to its extreme 

 southern point, we meet with the most diversified con- 

 ditions ; the most humid districts, arid deserts, lofty 

 mountains, grassy plains, forests, marshes, lakes, and 

 great rivers, under almost every temperature. There is 

 hardly a climate or condition in the Old World which 

 cannot be paralleled in the New — at least as closely 

 as the same species generally require ; for it is a most 

 rare case to find a group of organisms confined to any 

 small spot, having conditions peculiar in only a slight 



