GEOGRAPHICAL DISTRIBUTION 29 



The first fact geology proclaims is immense 

 number of extinct forms, and new appearances. 

 Tertiary strata leads to belief, that forms gradually 

 become rare and disappear and are gradually sup- 

 plied by others. We see some forms now becoming 

 rare and disappearing, we know of no sudden 

 creation : in older periods the forms appear to 

 come in suddenly, scene shifts: but even here 

 Devonian, Permian &c. [keep on supplying new 

 links in chain] Genera and higher forms come on 

 and disappear, in same way leaving a species on 

 one or more stages below that in which the form 

 abounded. 



(GEOGRAPHICAL DISTRIBUTION.) 



vi. Let us consider the absolute state of dis- 

 tribution of organisms of earth's face. 



Referring chiefly, but not exclusively (from 

 difficulty of transport, fewness, and the distinct 

 characteristics of groups) to Mammalia; and first 

 considering the three or four main [regions] divi- 

 sions; North America, Europe, Asia, including 

 greater part of E. Indian Archipelago and Africa 

 are intimately allied. Africa most distinct, especially 

 most southern parts. And the Arctic regions, which 

 unite N. America, Asia and Europe, only separated 

 (if we travel one way by Behring's St.) by a narrow 

 strait, is most intimately allied, indeed forms but one 

 restricted group. Next comes S. America, then 

 Australia, Madagascar (and some small islands which 

 stand very remote from the land). Looking at these 

 main divisions separately, the organisms vary accord- 

 ing to changes in condition 1 of different parts. But 

 besides this, barriers of every kind seem to separate 



1 In the Origin, Ed. i. p. 346, vi. p. 493, the author begins his discussion 

 on geographical distribution by minimising the effect of physical conditions. 

 He lays great stress on the effect of barriers, as in the present Essay. 



