UNITY AND DIVERSITY. 27 



found in the valleys that are most distant from each other. That is, 

 if the genus is present in all the valleys, the most divergent forms 

 will be found at the opposite ends of the mountain range, while in- 

 termediate forms will be found in the intermediate valleys. Allied 

 species occupying neighboring localities pass into each other by many 

 gradations of form and color, while those that are separated by a dis- 

 tance of 8 or 10 miles or more can not be connected by minute grada- 

 tions without bringing in some of the forms found in the intermediate 

 territory. 



Such are the main facts concerning these forms when viewed in their 

 relation to each other. L,et us now consider the external conditions. 

 Does the theory that all divergence of character is due to exposure 

 to diverse conditions find confirmation, or the opposite, in the facts 

 connected with these Hawaiian snails ? 



4. Diversity of Natural Selection not a Sufficient Explanation 0} the 

 Diversity of Species. 



Natural selection depends upon external conditions which are 

 either favorable or unfavorable to the success and propagation of 

 the organisms under consideration; and difference in natural selec- 

 tion must depend upon difference in the nature of the external condi- 

 tions that affect survival. Now, returning to the case of the nearly 

 allied arboreal species in the valleys near Honolulu, can we find any 

 diversity affecting survival in the conditions to which they are ex- 

 posed? In many cases we find them occupying the same species of 

 trees, and, accordingly, if their habits of feeding are different, we 

 must attribute this difference to spontaneous variation rather than 

 to the influence of external conditions. Extending our observations 

 to the climate and soil, we find the conditions unvaried, we might 

 well say, identical ; for the geological foundation of the whole moun- 

 tain range is volcanic basalt without the least limestone, and the soil 

 in these rugged valleys is not varied by the presence even of volcanic 

 ash, while the conditions of heat and moisture are the same on the 

 same side of the mountain crest. The vegetation in each valley is 

 essentially the same, and distributed in a similar way; for certain 

 shrubs and trees occupy the lower grounds and others the steep slopes 

 and ridges that rise above. If the species of mollusks which we are 

 studying have any enemies they are found in the forest regions 

 throughout the island. No insect or reptile, no bird or mammal, 

 found on the island is limited to a district of only 2 or 3 miles extent. 



As we fail to find any cause for the divergences of character in the 

 external conditions, we must believe either that the conditions are 



