GEOGRAPHICAL DISTRIBUTION, 373 



CHAPTER XII. 



GEOGRAPHICAL DISTRIBUTION. 



Present distribution cannot be accounted for by differences in 

 physical conditions — Importance of barriers— Affinity of the 

 productions of the same continent — Centers of creation— Means 

 of dispersal by changes of climate and of the level of the lan.l, 

 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 whicli 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, m.arshes, 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 so closely as tlie 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 fauiia different from that of tlie 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 thia 



