28 THE EVOLUTION OF NATURAL SPECIES. 
more diversified than they appear to be, or that the causes of change 
in these cases belong wholly to the nature of the organism. That the 
former supposition is not the true one seems to me to be sufficiently 
proved by the following facts and considerations: 
(1) If the divergence of character was due to difference of natural 
selection resulting from difference in external conditions, we should 
expect that the distribution of species would bear some marked rela- 
tion to the amount of rainfall, which, owing to the northeast trade 
winds, is considerably greater on the northeast side of the mountain 
range than on the southwest side. On the contrary, we find that 
species of the same group, found in valleys on the same side of the 
range, but 30 or 35 miles apart, are far more divergent from each 
other than are those found on opposite sides of the range but at 
points much nearer. Assuming that they have migrated from a com- 
mon center, the distance between two species measures in a rough 
way the relative number of generations that have passed since their 
ancestors parted company, and the degree of divergence is in propor- 
tion to the time and degree of separation rather than in proportion to 
the degree of difference in external conditions. 
(2) This law of distribution is found, not simply in the case of a 
few related forms, in which it might be supposed that the diversity 
of conditions was real, though not apparent, but is the general law 
according to which the 200 species and 800 or 1,000 varieties of Acha- 
tinellida found on this island are for the most part distributed. This 
law also controls the distribution of mollusks on the neighboring 
islands and in many parts of the world. 
(3) If we assume that the divergence is due to the diversity of 
natural selection, we must hypothecate a series of conditions affecting 
survival presenting increasing differences with each additional mile.* 
5. The Causes of the Divergence must lie in the Independent Action of 
the Segregated Sections of the Original Stock. 
For my part I find it easier to believe that the causes of divergence 
of character in these forms that are, so far as we can discover, exposed 
to the same conditions, exist in the organism. 
(1) We know that the individual variations in specimens of one 
species found on one tree are often very considerable. 
(2) Unless the degree and kind of variation is znvarzable in sepa- 
rated groups of the same species, it is self-evident that there must be 
a tendency to divergence of character. 
* See Plate II, figs. 11-25. 
