28 THE EVOLUTION OF NATURAL SPECIES. 



more diversified than they appear to be, or that the causes of change 

 in these eases 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: 



(i) 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- 

 tinellidae 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 invariable 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. 



