4 INTRODUCTION. 



complete explanation of divergence, even for higher animals; and in 

 the case of creatures as low as mollusks it would seem to be entirely 

 excluded from having any effect in determining the diversity of color 

 in the different species. 



2. In Many of the Same Cases Diversity of Natural Selection can not 



be the Cause. 



First. Because in many cases divergence is not in proportion to the 

 degree of difference in the environments surrounding the separated varie- 

 ties and species. This is true not only in regard to the divergence of 

 genera and subgenera of snails, occupying different islands of the 

 Hawaiian group, but in regard to the divergence of the varieties and 

 species of the same subgenus, occupying the different districts of 

 the same island. Darwin's theory assumes that when a few members 

 of a species form a colony in a new district divergence is produced 

 only when, and in proportion as, the new district presents condi- 

 tions unlike those found in the original habitat of the species. This 

 interpretation fails to explain the origin of the species we are now 

 considering. For example, valleys separated by narrow mountain 

 spurs, on the southwest side of the main mountain range of the island 

 of Oahu, are exposed to similar wind, rain, and temperature ; the soil 

 has come from the disintegration of volcanic rock, without limestone ; 

 the vegetation on the ridges differs from that in the valleys; but in 

 most of the valleys we find groves of candlenut trees (Aleurites tri- 

 loba), and clinging to the trunks and branches of the trees in these 

 groves in any one valley we find several species and many varieties 

 that are not exactly reproduced in any valley more than 2 or 3 miles 

 distant. The valleys of Manoa and Nuuanu are but 3 miles apart, 

 but they present a greater difference in vegetation than that found 

 between Manoa and Kawailoa, which are 20 miles apart ; the diver- 

 gence in species of Achatinella occupying these valleys is, however, 

 much less in the former case than in the latter. This is the reverse 

 of what we should find if the divergence were due to exposure to unlike 

 conditions. This is not an exceptional case. The land mollusks of 

 the Hawaiian Islands present a vast body of facts of this kind. 



Second. Because in some cases the divergence is in non-utilitarian 

 characters. If these snails were endowed with powers of vision and 

 discriminating instincts as highly developed as those of birds, it 

 might be argued that the brilliant colors characterizing certain 

 species were developed by what I have called social segregation, with 

 the aid of sexual and social selection, including the need of recognition 

 marks, which Mr. Wallace has pointed out as a prime necessity in the 

 segregation of higher animals; but, as we have no reason to think 



