UNITY AND DIVERSITY. 2a Ff 
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. Let 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 of 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 
