310 FROM THE GREEKS TO DARWIN 



The individuals of genera, on continents, spread 

 and widely diffuse themselves, and owing to differ- 

 ences of localities, nourishment, and soil, form vari- 

 eties ; and in consequence of their isolation and never 

 being crossed by other varieties and so brought back 

 to the main type, they, in the end, become a perma- 

 nent and distinct species. Then, perhaps, in other 

 ways, they meet with other descendants of the orig- 

 inal form — which have likewise become new varieties 

 — and both now appear as very distinct species, no 

 longer mingling with one another. Not so on islands. 

 Being commonly confined in narrow valleys, or within 

 the limit of small zones, individuals can reach one 

 another and destroy every commencing production 

 of a permanent variety. 



We find in von Buch a clear conception of the 

 force of geographic 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 Buf- 

 fon and St. Hilaire. 



Haldeman (1812-1880) 



In 1844 Haldeman gave a full discussion of 

 the arguments for and against the *Lamarckian 

 hypothesis,' in a paper^ entitled Enumeration 

 of the Recent Fresh-water Mollusca which are 

 Common to North America and Europe, He 



"^Bost. Journ. Nat. HisU 184S-4. 



