EVOLUTION THEOUGH CUMULATIVE SEGREGATION. 189 



Divergent Evolution through Cumulative Segregation. By Eev. 

 J OHN Thomas Gultck. (Communicated by Alfeed Eussel 

 Wallace, F.L.S.) 



[Eead 15th December, 1887.] 

 Inteoduction. 



In my study of Sandwich-Island terrestrial mollusks my atten- 

 tion was early arrested by the fact that wide diversity of allied 

 species occurs within the limits of a single island, and in 

 districts which present 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 im- 

 possible that the form and colouring 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 diver- 

 gence 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 perceptibly 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 favourable to migration. I perceived that 

 these facts could all be harmonized by assuming that there is some 

 cause of divergence more constant and potent than differences in 

 nature external to the organism ; and that the influence of this 

 cause was roughly measured by the time and degree of separation. 



During the summer of 1872 I prepared two papers in which 

 these facts and opinions were presented. One of these, entitled 

 "The Variation of Species as related to their Geographical Dis- 

 tribution, illustrated by the Achatinellinae," was published in 



LINN. JOUEN. — ZOOLOGY, VOL. XX. 16 



