CnAf. XII.) GEOGRAPHICAL DISTRIBUTION. 129 



CHAPTER XII. 



GEOGEAPHICAL DISTKIBUTION. 



Present distribution cannot be accounted for by difEerences 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 dissimi- 

 larity 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 



