SELECTION NOT ALWAYS THE CAUSE. 3 
that are separated by the greatest distance, which also corresponds 
with separation by the greatest number of valleys and mountain 
spurs. 
I had read Darwin’s account of the Galapagos Islands given in 
““The Voyage of the Beagle,’’ and had noted the astonishment with 
which he had discovered that islands within sight of each other were 
the homes of closely related but diverse species—a marvel which 
he in no way attempted to explain; and here, in the snails of 
Hawaii, I had found differences more wonderful, for each valley of 
the same island was the home of peculiar forms. When Darwin’s 
“Origin of Species” appeared in 1859 I read it with intense: interest 
and with complete assent to his argument that large groups of widely 
divergent species had been derived from common ancestry; but the 
more I meditated on the scope of natural and sexual selection as fac- 
tors producing transformation, the more clearly I perceived that they 
were not adequate to explain the diversity of these species exposed to 
the same external conditions; neither did they afford any explana- 
tion of why the areas of distribution for many of these species are so 
extremely limited, while some species of terrestrial mollusks are dis- 
tributed over vast areas. 
II. Facts SHOWING THAT IN THE CASE OF MANY DIVERGENT SPECIES DIVERSITY 
OF NATURAL AND SEXUAL SELECTION IS EITHER WANTING OR, IF PRESENT, 
IS THE EFFECT AND NOT THE CAUSE OF THE DIVERGENCE.* 
1. In Many Cases of Divergence Diversity of Sexual Selection can not 
be the Cause. 
In the case of mollusks, the diversity in colors and forms presented 
by closely allied varieties and species can not be attributed to diver- 
sity in the direct action of the sexual instincts of the different groups, 
by which those of certain forms and colors are allowed to mate, to the 
exclusion of all other forms and colors; for there is no reason to 
believe that differences of form and color are capable of being at all 
observed by the senses with which they are endowed. Even in the 
case of highly endowed animals, where diversity in the styles of orna- 
mentation desired in mates, and therefore diversity in the forms of 
sexual selection in closely allied varieties and species, may be readily 
granted, the problem of chief interest is not concerning the effect of 
these divergent instincts, but rather concerning the causes by which 
they have been produced; for why should an isolated section of a 
species possess instincts in any degree differing from the instincts of 
the main body of the species? Sexual selection is, therefore, an in- 

* In Chapter IV will be found illustrations of divergence under the same envi- 
ronment. 
