CHAPTER XII 

 Geographical Distribution 



Present distribution cannot be accounted for by differences in physi- 

 cal conditions — Importance of barriers — Affinity of the produc- 

 tions 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 conclu- 

 sion. 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 fun- 

 damental divisions in geographical distribution is that be- 

 tween 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 can- 

 not be paralleled in the New — at least as closely as the same 

 species generally require. No doubt small areas can be 

 pointed out in the Old World hotter than any in the New 

 World; but these are not inhabited by a fauna different from 

 that of the surrounding districts; for it is rare to find a group 

 of organisms confined to a small area, of which the con- 

 ditions are peculiar in only a slight degree. Notwithstand- 

 ing this general parallelism in the conditions of the Old and 

 New Worlds, how widely different are their living pro- 

 ductions ! 



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