GEOGRAPHICAL DISTRIBUTION 187 



CHAPTER XII 



GEOGRAPHICAL DISTRIBUTION 



Present distribution cannot be accounted for by differences in physical 

 conditions — Importance of barriers — Affinity of the productions of tlie 

 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 dis- 

 similarity 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 divi- 

 sions 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 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 

 cannot be paralleled in the New — at least as closely as 



