CHAPTER IV. 



DIVERGENCE UNDER THE SAME ENVIRONMENT. 



Explanation of Plate I. 



Plate I presents 2 1 species of 8 genera of one family found on five of 

 the islands of the Hawaiian group. These islands are within sight of 

 each other, having the same climate and much the same vegetation, 

 and (with the exception of the different forms of snails) the same spe- 

 cies of animal life ; and yet how great the diversity presented by the 

 species of the same genus, not to mention the greater divergence 

 attained by the different genera. These eight genera, and the two 

 genera represented in Plates II and III, all belong to the family of 

 Achatinellidae, found in no part of the world outside of the Hawaiian 

 Islands. These genera, though differing widely in form and habits, 

 have one shell-characteristic which does not occur in the same degree 

 in snail shells found in other parts of the world. It will be observed 

 that the small glassy genus Leptachatina (fig. 5), and the minute Auri- 

 culella, with a sharp plate in the aperture (fig. 3), are like the much 

 larger shells of the other genera, in that they have a twist in the colu- 

 mella. The character appears in a greater or less degree in all the ten 

 genera by which the Achatinellidae are represented. 



Of these eight genera, Carelia is found only on Kauai, Apex and 

 Bulimella only on Oahu, while Amastra (and probably Leplachatina), 

 is found on all the isiands of the group, and Auriculella, Laminella, 

 and Partulina are found on the central islands, that is, on Oahu, Molo- 

 kai, Landi, and Maui. The one arboreal species I have received from 

 the island of Hawaii is either a PartuUna or a Newcombia, the specific 

 name being physa (Newcomb). The typical forms of Newcombia are 

 found on Maui and Molokai. Laminella and Partulina find special 

 development on Maui, Molokai, and Lanai, where they are represented 

 by many species. The genera most characteristic of the island of Oahu 

 are BuUmella, Achatinella, and Apex. The most nearly aUied species 

 from different islands do not completely intergrade as do those from 

 the same island. For example, Partulina splendida of Maui (fig. 14) 

 does not completely intergrade with P. virigulaia of Molokai (fig. 12), 

 or with P. variabilis of Lanai (fig. 13), as it does with certain other spe- 

 cies of Partulina found on Maui. The species of this genus have been 

 so chosen as to present from each island the form most nearly related 

 to PartuUna splendida. So also in the case of the Laminella, it is not 



37 



