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THE AMERICAN NATURALIST. [Vol. XXXVIII. 



of the fundament of the nerve cord in Ph. gonldii, immediately 

 before metamorphosis, points definitely to an intimate relation- 

 ship between the sipunculids and the annelids. Sipunculiis is 

 more highly modified than Phascolosoma. 



Divergence Under the Same Environment as Seen in the 

 Haivaiian Snails: John T. Gulick, Oberlin, Ohio.— The 

 Island of Oahu presents two mountain ranges ; one near the 

 northeast coast, about 35 miles in length, and enjoying a heavy 

 rainfall, especially on the side of the range most exposed to the 

 northeast trade-winds ; the other, near the southwest coast, 

 about half as long, and receiving much less rain. The forest 

 regions on these two mountain ranges are occupied by about 

 300 species of snails belonging to eight genera of the family 

 Achatinellidas. A large majority of these species belong to four 

 genera found always on the trees or other vegetation. Auricu- 

 lella clings to the leaves, is about the size of a grain of wheat, 

 and is represented by one abundant species on the dry mountain 

 range, and by several species on the other range. Apex is from 

 half an inch to an inch in length, and occupies the dry range, 

 and the drier ridges of the other range. Achatinella and 

 Hulimella are in size not very different from Apex, and like it 

 are found on the trunks and branches as well as on the leaves ; 

 but their distribution is almost entirely on the longer mountain 

 range, Achatinella being more abundant on the less rainy side, 

 and Bulimella prevailing on the more rainy side. 



Natural selection has undoubtedly been an important factor 

 in adapting Bulimella for the rainiest districts ; and Achatinella 

 and Bulimella for the shady groves of the less rainy region, and 

 Apex for the driest region as w^ll as for the deficient shade on 

 the ridges of the intermediate region. 



But we come now to species whose divergence refuses to be 

 explained by either natural selection or sexual selection. As an 

 example of divergent evolution that must depend on other con- 

 trolling principles, let us consider the formation of the series of 

 species of Achatinella occupying the valleys on the southwest 

 side of the main mountain range. The temperature, the rain- 

 fall, the soil, and the plant and animal species surrounding these 

 snails are much the same. 



