198 ADAPTATION 



pose it. In nearly all cases where a species is 

 distributed over a wide, discontinuous unit of 

 environment, i.e. an area broken up into isolated 

 parts, the parts contain forms that are meas- 

 urably different from each other. 



A most instinctive example is furnished by 

 the Characins. Astyanaac fasciatus is found 

 from Patagonia to Mexico, except at Panama 

 and the Rio Parahyba. It differs in different 

 localities, and in the Rio Parahyba, near Rio de 

 Janeiro, and at Panama the differences have be- 

 come of specific value. The species is continued 

 in southern Mexico as Astyanax ceneus, and in 

 northern Mexico as Astyanaao argentatus. In 

 other words, in those cases where the divergence 

 has gone far enough we call the divergents spe- 

 cies, in those cases where they are diverging, vari- 

 eties. These geographical varieties are species 

 in the making, just as truly as the elementary 

 species of De Vries.^ 



Isolation is not always accompanied by differ- 

 entiation. Some species of Galaxias in Patago- 

 nia and Australia are identical, while those in 

 different parts of Patagonia are different. Geo- 

 graphical isolation must lead to differentiation 

 if the isolation forces the individuals to live in 

 places on the whole different from their original 

 home. A species {Astyanax fasciatus) may be 

 all but identical even if isolated in different rivers 



' The different diverging lines will be fully considered in my 

 monograph on the Characins, now in preparation. 



