346 KEY. J. T. aULICK OS 



succeeding generations than those that are sterile; and of fertile 

 families, those that are above the average in intellect will have the 

 best success in leaving descendants to inherit their endowments. 



Combined IisrELUEJsrcE of these Principles. 

 We have not at present sufficient knowledge of the influence 

 of each of the principles of transformation to enable us to esti- 

 mate their comparative importance ; but we know enough of 

 their combined action to anticipate with confidence that wherever 

 Separate or Segregate Greneration arises, producing more or 

 less divergence, there these principles 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 comprehending the 

 process we are compelled to study each principle by itself, we 

 must remember that in nature they not only combine, but com- 

 bine in a vast variety of ways. There is, however, reason to 

 believe that species sometimes become so devoid of plasticity that 

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

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



Diteb&ent Evolution in the Land-Mollusks oe 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 north-east and the 

 other on the south-west side of the island, show no signs of recent 

 volcanic action. Unlike the mountains of Hawaii and East Maui, 

 their sides are very deeply furrowed by the action of water, and 

 their forests are not broken by flows of lava. The forests of the 

 island cover these two ranges, forming two disconnected strips, 

 the one about 36 and the other about 18 miles in length. In 

 these forests are found 600 or 700 varieties, representing over 

 200 species, belonging to 7 subgenera, of the subfamily Achati- 

 nelUncB. 



Two of these subgenera, Amastra and LeptacJiatina, are, for 

 the most part, found under the dead leaves of trees in damp 

 places ; and one, Laminella, is found chiefly on low shrubs, while 

 the remaining four are always found on trees or shrubs. Now 

 it must be remembered that the climate is tropical, and that the 



