" DIE STAMME DES THIERREICHS 65 



tains a very great number. Each of these species is 

 strictly confined to one of the wooded ravines which 

 exist everywhere in this island. Nearly every 

 ravine contains its species, and the closer the ravines 

 are to each other, the closer is the resemblance be- 

 tween their respective species. Between the species 

 of each island there exist hardly distinguishable 

 transitional forms which are wanting in the case of 

 species belonging to different islands. Now, quite 

 recent circumstances have determined the complete 

 extinction of a great number of the species of Oahu, 

 so that the former continuity of these forms is to- 

 day broken. At the present moment the Achatin- 

 ellse of the Island of Oahu are no longer represented 

 save by a small number of species, very distinct 

 from each other, without transitional forms, just as 

 if they were species belonging to different islands. 

 Thus, by chance we have been able to see the actual 

 operation of a natural process of dissociation of 

 species by the extinction of intermediate varieties. 

 This allows us to understand that types now clearly 

 distinct and separated may be only the survivors of 

 a formerly continuous series. 



But what are the causes that can have led to the 

 variation of this primitive series ? According to 

 Neumayr, this variability is closely connected with 

 the habitat, that is, the changes produced in the 

 conditions of existence of the animal. He finds, 

 as did Darwin, very remarkable examples in the 

 changes of fauna which take place in islands 

 isolated in the bosom of great oceans. These 

 islands, generally of volcanic origin or of reef forma- 

 tion, and in consequence somewhat recent, can only 



F 



