322 GEOGKAPIIICAL DISTRIBUTION. Cuai-. XI. 



CHAPTER XI. 



GKOGRAPIIICAL DISTKiCUTION. 



f resent Distribution cnnnot be accounted for by Differences in Phj-sical Conditions 

 — Importance of Barriers — Afllniiy of the Prodnctiotis 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 accoimted for by their climatal and 

 other jihysical conditions. Of late, almost every author who 

 has studied the subject has come to this conclusion. The case 

 of America alone would almost sulllce to prove its truth : for, if 

 we exclude th.e northern parts where the circumpolar land is 

 almost continuous, all authors agree that one of the most fun- 

 damental divisions in geographical distribution is that between 

 the New and Old Worlds ; yet, if Ave travel over the vast 

 American Continent, from the central parts of the United 

 States to its extreme southern point, we meet with tin; 

 most diversified conditions ; liinnid districts, arid deserts, lofty 

 moimtains, grassy plains, forests, marshes, lakes, and great 

 rivers, under almost every temperature. Tlierc is hardly a 

 climate or condition iii the Old World which cannot be paral- 

 leled in the New — at least as closely as the same species gen- 

 erally require. No doubt small areas can be jiointed out in 

 the Old A\''orld hotter than any in the New A^'orld, but these 

 are not inhabited by a fauna different from that of the sur- 

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

 isms conilned to a small area, having conditions peculiar in 

 only a slight degree. Notwithstanding this general parallel- 

 ism in the conditions of the Old and New Worlds, how widely 

 different are their living productions! 



In tho southern hemisphere, if wc compare large tracts of 



