Chap. XII.] GEOGKAPHICAL DISTRIBUTION. 129 



CHAPTEE XII. 



Geographical Distribution. 



Present distribution cannot be accounted for by differences in 

 physical 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 — 

 Alternate Glacial periods in the North and South, 



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 wholly 

 accounted for by 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 arctic and northern temperate parts, 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 conditions; 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 



