214 DARWIN. 



" The individuals of genera on continents spread and widely 

 diffuse themselves; owing to differences of localities, nourishment, 

 and soil, they form varieties ; and in consequence of their isola- 

 tion and never being crossed by other varieties and so brought 

 back to the main type, they, in the end, become a permanent 

 and distinct species. Then, perhaps, in other ways, they meet 

 with other descendants of the original form, — which have likewise 

 become new varieties, — and both now become distinct species, no 

 longer mingling with one another. Not so on islands. Being 

 commonly confined in narrow valleys, or within the limits of small 

 zones, individuals can reach one another and destroy every com- 

 mencing production of a permanent variety." 



We find in Von Buch a clear conception of the 

 force of Isolation or Segregation, which had been 

 observed by Buffon, as we have seen ; his theory 

 of Evolution is also that of the direct action of 

 environment, advocated by Buffon and St. Hilaire. 



In 1844 {Bosi. Journ. Nat. Hist., 1843-44), Hal- 

 DEMAX gave a full discussion of the arguments for 

 and against the ' Lamarckian hypothesis,' in a paper 

 entitled " Enumeration of the Recent Fresh-water 

 MoUusca which are Common to North America 

 and Europe." He wrote, apparently, from Lyell's 

 exposition of Lamarck, rather than from the original 

 author himself. He inclined strongly to the trans- 

 mutation theory, although hesitating to offer a 

 direct opinion. As to the causes of modification, 

 he ignores Lamarck's special theory, and tends 

 rather to adopt Buffon's factor of the direct action 

 of the environment. 



Herbert Spencer appeared as one of the few 



L 



