CHAPTER XII 

 GEOGRAPHICAL DISTRIBUTION 



Present Distribution cannot be acx:ounted 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 — ^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 re- 

 gions can be wholly accounted for by climatal and other physical 

 conditions. Of late, almost every author who has studied the sub- 

 ject 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 be- 

 tween the New and the 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 moun- 

 tains, grassy plains, forests, marshes, lakes and great rivers, 

 under almost every temperature. There is hardly a climate or con- 

 dition in the Old World which cannot be paralleled in the New — 

 at least so 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 differ- 

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

 group of organisms confined to a small area, of which the condi- 

 tions are peculiar in only a slight degree. Notwithstanding this 

 general parallelism in the conditions of Old and New Worlds, 

 how widely different are their living productions! 



In the southern hemisphere, if we compare large tracts of land 

 in Australia, South Africa, and western South America, between 

 latitudes 25 and 35 degrees, we shall find parts extremely 

 similar in all their conditions, yet it would not be possible to point 



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