212 APPENDIX ii INTENSIVE; SEGREGATION. 



segregations of a partial kind ; and the different degrees of intercross- 

 ing between the minor segregations in the separate districts would be 

 an additional cause of divergence, which we may appropriately class 

 as a form of amalgamational intension. Occasional interchange of 

 stations by the varieties in one district would produce a degree of 

 homogeneity in the forms of one district that would not be found when 

 comparing those of different districts; but as the degrees of inter- 

 crossing between any two or more identical varieties that might 

 happen to be preserved in both districts would, in all probability, differ 

 in different districts, the correspondence that at first existed between 

 certain portions of the two sections would gradually disappear. 



11. Combined Influence of these Principles. 



We have not at present sufficient knowledge of the influence of each 

 of the principles of transformation to enable us to estimate their com- 

 parative importance ; but we know enough of their combined action 

 to anticipate with confidence that wherever separate or segregate 

 generation arises, producing more or less divergence, there these prin- 

 ciples will in time intensify the result. The transformations and 

 divergences of nature are produced by the interplay of numerous 

 factors most intimately combined, and though for the purpose of com- 

 prehending the process we are compelled to study each principle by 

 itself, we must remember that in nature they not only combine, but 

 combine in a vast variety of ways. There is, however, reason to be- 

 lieve that species sometimes become so devoid of plasticity that nearly 

 all transformation is precluded, and, if the environment is greatly 

 changed, even in the most gradual manner, extinction is the result. 



II. DIVERGENCE IN MOLLUSKS. 

 1 . Divergent Evolution in the Land Mollusks of Oahu. 



Oahu is one of the Sandwich Islands, or Hawaiian Islands, as they 

 are now usually called. It is of volcanic origin, but the two mountain 

 ranges, which lie one on the northeast and the other on the southwest 

 side of .the island, show no signs of recent volcanic action. Unlike the 

 mountains of Hawaii and East Maui, their sides are very deeply fur- 

 roWed by the action of water, and their forests are not broken by 

 flows of lava. The forests of the islands cover these two ranges, form- 

 ing two disconnected strips, the one about 36 and the other about 18 

 miles in length. In these forests are found many hundred varieties, 

 representing over 200 species, belonging to 8 genera, of the Achati- 

 nellidae. 



