DIVERGENT EVOLUTION THROUGH CUMULATIVE SEGRE- 
GATION.* f 
By Rev. JoHN THOMAS GULICK. 
INTRODUCTION. 
In my study of Sandwich Island terrestrial mollusks, my attention 
was early arrested by the fact that wide diversity of allied species 
occurs within the limits of a single island and in districts which pre- 
sent essentially the same environment. As my observations extended 
I became more and more impressed with the improbability that these 
divergences had been caused by differences in the environment. It 
was not easy to prove that sexual selection had no influence; but, 
owing to the very low grade of intelligence possessed by the creatures, 
it seemed impossible that the form and coloring of the shells should be 
the result of any such process. I was therefore led to search for some 
other cause of divergent transformation, the diversity of whose action 
is not dependent on differences in nature external to the organism. 
I found strong proof that there must be some such principle, not 
only in the many examples of divergence under uniform activities in 
the environment, but in the fact that the degrees of divergence between 
nearly allied forms are roughly measured by the number of miles by 
which they are separated, and in the fact that this correspondence 
between the ratios of distance and the ratios of divergence is not per- 
ceptibly disturbed by passing over the crest of the island into a region 
where the rainfall is much heavier, and still further in the fact that the 
average size of the areas occupied by the species of any group varies, 
as we pass from group to group, according as the habits of the group 
are more or less favorable to migration. I perceived that these facts 
could all be harmonized by assuming that there is some cause of diver- 
gence more constant and potent than differences in nature external to 
the organism, and that the influence of this cause was roughly meas- 
ured by thé time and degree of separation. 
During the summer of 1572, I prepared two papers, in which these 
facts and opinions were presented. One of these, entitled ‘* The Vari- 
* [Read December 15, 1887.] From The Journal of Zodlogy of the Linnean Society, 
September, 1888, vol. Xx, pp. 189-274. Ee 
