CHAPTER XI 



GEOGRAPHICAL DISTRIBUTION 



Present distribution cannot be accounted for by differences in physical 

 conditions— Importance of barriers— Affinity of the productions 

 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 th6 

 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 distribu- 

 tion 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 co 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 



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