350 GEOGRAPHICAL DISTRIBUTION. 



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 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 the Old Worlds ; 

 yet if we travel over the vast American continent, from 

 the central parts of the United States to its extreme south- 

 ern 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 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 different from that of the sur- 

 rounding districts ; for it is rare to find a group of organ- 

 isms confined to a small area, of which the conditions 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 



