26 THE EVOLUTION OF NATURAL SPECIES. 



are usually variable, that is, polymorphic, and that these same genera 

 are the ones in which the production of new species is most rapidly 

 progressing ; but he has nowhere drawn the conclusion that freedom 

 from rigid natural selection can in any way favor the production of 

 new varieties and species. On the contrary, he teaches that it is only 

 through the agency of diversity in natural selection that individual 

 variations can be accumulated in diverging lines that become more 

 and more distinct. (See Origin of Species, Chapter IV.) 



3. Facts in the Distribution of Hawaiian Snails. 



It was through the study of island fauna that I was first led to 

 doubt the correctness of Darwin's theory at this point. The terres- 

 trial mollusks of the West Indies present important differences as we 

 pass from island to island, but it was in the Hawaiian Islands that I 

 found the greatest difference in the species inhabiting the forests in 

 different parts of the same island. The remarkable features in the 

 distribution and affinities of these forms will be most easily presented 

 by giving a brief statement of some of the facts relating to those 

 found on the island of Oahu. This island, about 45 miles in length 

 and 20 miles in width, is inhabited by over 200 species of land snails, 

 represented by 800 or i ,000 varieties ; and these are nearly all confined 

 to the forests covering two ranges of mountains, the one 1 5 or 20 miles 

 and the other 35 miles in length. But the most remarkable fact is not 

 the great number of species and varieties inhabiting this small area, 

 nor yet that all of them (with, perhaps, one or two exceptions) are 

 peculiar to this island ; but that each of these forms is confined to only 

 a small section of this small area. Not only are the species on each 

 of the ranges of mountains different, but those found on one range 

 and inhabiting one continuous region of forest are not distributed at 

 raijdom over that region. On the contrary, each valley not more than 

 half a mile in width and perhaps two miles in length has its own 

 peculiar varieties, and in some cases its own species, which are found 

 nowhere else. 



As the explorer passes from valley to valley he will find six or seven 

 quite distinct groups of forms, each group being a genus or a sub- 

 genus of the one family of Achatinellidae. Taking any one genus and 

 tracing its distribution from valley to valley, he will find that, as far 

 as it extends, it is represented in each valley by one or more closely 

 allied species, each species being represented by several varieties. 

 One species may be confined to a single valley, or it may extend over 

 several, being represented in each by varieties peculiar to that valley. 

 The more widely divergent forms of one group or genus will be 



