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 1n many cases divergence ts not in proportion to the 
degree of difference in the environments surrounding the separated varte- 
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 sameisland. 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 
