EXPLANATION OF PLATE II. 39 



is probably from Kalihi. No other species from the Hawaiian Islands seems to 

 so closely resemble the Partula found on many of the islands of the Pacific. Still 

 it must belong to the Achatinellidae, for it completely intergrades with Partulinu 

 densilineata, which presents the peculiar twist of the columella, the most constant 

 shell-character of the Hawaiian family of snails.* 



FIG. 12. Partulina virigulata (Nwc.) D > S. From the island of Molokai. 



FIG. 13. Partulina -variabilis (Nwc.) S. From the island of Lanai. 



FIG. 14. Partulina splendida (Nwc.) D > S. From Lahaina, West Maui. 



FIG. 15. Partulina plumbea (Gk.) D. From Kula, East Maui; found on the 

 trunks of trees several thousand feet above the sea level. 



FIG. 1 6. Newcombia cumingii (Nwc.) S. From Makawao, East Maui. 



FIG. 17. Amastra turritella (Per.) D. Found on dead trees from Waiawa east- 

 ward to Keawaawa, island of Oahu. This specimen is from Palolo. 



FIG. 1 8. Amastra -violacea (Nwc.) D. From the island of Molokai. 



FIG. 19. Amastra magna (Adams) D. From the island of Lanai. 



FIG. 20. Amastra mastersi (Nwc.) D. From Lahina, West Maui. 



FIG. 21. Amastra nigra (Nwc.) D. From Kula, East Maui. 



The Darwinian theory explains the unity of form in the different 

 genera and species of one family by the theory that they are all de- 

 scended from one original intergenerating stock. The diversity in the 

 forms is explained as the result of variation, with diversity in the forms 

 of selection produced by exposure to different environments. In explain- 

 ing the divergence in the genera of this family, I think, we shall come 

 nearer to the facts if we attribute the diversity in the selection, to 

 which they have been exposed for countless generations, to diversity 

 in the methods of using the environment adopted by completely isolated 

 groups. The divergence in many of the closely related species, found 

 on the same species of trees in successive valleys on the same moun- 

 tain range, is, I think, due to isolation and variation, without any differ- 

 ence in the forms of selection. 



EXPLANATION OF PLATE II. 



This plate presents 25 species of Achatinella, a genus of arboreal 

 snails found on the island of Oahu, and most of them confined to dis- 

 tricts from i to 3 or 4 miles in extent. Achatinella is one of ten genera 

 of the Achatinellidse, of which there are between 200 and 300 species 

 and over a thousand varieties, on this island only 40 miles long. 

 Bight of these genera are given on Plate I, and one on Plate III. This 

 plate illustrates the relationship of species distributed in different 

 valleys on the same island. The letter attached to each figure desig- 

 nates the valley or small district in which the specimen figured was 

 found. The position of this valley or district on the island is easily 



* A careful comparison of the internal structure of this species with the struc- 

 ture of the Partula of the South Pacific might perhaps reveal points of special 

 interest in the relations of the two groups. 



