S16 GEOGRAPHICAL DISTRIBUTION. TCiiap. XIL 



CHAPTEE XII. 



Geographical Distribution. 



Fresent 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 — Alternate Glacial periods in the 

 North and South. 



In considering the distribution of organic beings over tbe 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 con- 

 ditions. 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 moun- 

 tains, 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. 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 conditions are 

 peculiar in only a slight degree. Notwithstanding this general 

 parallelism in the conditions of the 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 lati- 

 tudes 25° and 35°, we shall find parts extremely similar in all their 



