40 DIVERGENCE UNDER THE SAME ENVIRONMENT. 



discovered by reference to the map of Oahu preceding Plate II, where 

 the same letters are used to designate positions. It will be observed 

 that underscored letters are used to designate positions on the north- 

 east side of the main mountain range, and that Roman letters are used 

 to designate the positions on the southwest side of the same range. 

 These are so arranged that the valleys indicated by the two forms of 

 any <me letter are nearly opposite to each other. 



The twenty live species presented in this plate would have to be 

 considered as one species if we accepted the statement that the find- 

 ing of completely intergrading forms between two types proves that 

 thev are varieties of the same species. If, on the other hand, we 

 adopt the statistical method of testing species,* these, and many more 

 found in the forests of the same mountain range, will have to be classed 

 as species. The collection of the species and varieties of this genus, 

 made by me from 1850 to 1852, present a complete series of intergra- 

 dations between the larger forms found on the trunks of the candle- 

 nut tree in the eastern valleys of Oahu, and the small forms found on 

 the leaves of the lobelia and other shrubs in the western valleys of the 

 island, where groves of the candle-nut tree abound, occupied more or 

 less by species of Bulimella, but neglected by the representatives of 

 the genus Achatinella. As no one will maintain that these most diver- 

 gent types of the genus belong to the same species, the existence of 

 such a series of intergrading links becomes a strong argument for the 

 belief that divergent forms properly ranking as species have arisen 

 from one original species, through the cumulative effects of variation 

 cooperating with a series of isolations, each isolation lasting for many 

 generations before the next occurs. 



The genera most characteristic of Oahu are Achatinella, Bulimella, 

 and Apex; for, with the exception of two or three species of Achati- 

 nella found on the island of Molokai, they are limited in their distribu- 

 tion to this island. Their distribution in the different districts of the 

 island is probably due to the adaptation of Bulimella to a damp cli- 

 mate, and of Achatinella to a region of dense shade, but of less rain, 

 and of Apex to a comparatively dry climate. As regards rainfall, the 

 northeast side of the main range catches the trade winds as they come 

 from the ocean, and receives the- heaviest fall; on the southwest side 

 of the same range there is less rain; and on the shorter range, on the 

 southwest side of the island, there is still less rain. In the first region 

 we find Bulimella and a very few Achatinella; in the second region 



* The explanation of Plate III brings out the fact that the individuals represent- 

 ing intermediate forms are very rare compared with those of typical forms. Such 

 species meet the statistical test. 



