Xll INTRODUCTION. 



or rejuvenescence of the old stock in comparatively modern 

 (Tertiary) time. The aspect of youthful exuberance in the 

 Achatinellid fauna is remarkable. Phylogerontic or aged 

 groups, such as are usually common in old island faunas, are 

 conspicuously wanting. Everywhere intense local differen- 

 tiation is in progress. 



II. 



The Hawaiian Islands are, as is well known, volcanic masses 

 standing upon the southeastern end of a submarine ridge, 

 over 1,700 miles in length, stretching from Ocean and Mid- 

 way Islands to Hawaii, and rising from a depth of about 

 3,000 fathoms. The present islands being wholly volcanic, so 

 far as known, many geologists have assumed that the entire 

 ridge has been built up of volcanic materials. This inference 

 is unsupported by evidence. The Andean ridge is not wholly 

 volcanic because it is crowned by great volcanoes. It is quite 

 possible that the Midway-Hawaiian ridge is a product of dia- 

 strophism which preceded the volcanic period. However this 

 may be, the richness and peculiarity of the fauna and flora, 

 and the belief that the volcanic islands as they now stand are 

 probably of no great age, has led to the hypothesis that for- 

 merly a much greater land area existed, now lost by subsi- 

 dence. So conservative a zoogeographer as Wallace considers 

 this probable ; and except for an advocate of special creation, 

 the theory of a land area antecedent to the present volcanic 

 islands seems necessary to account for the faunal character- 

 istics. That there has been a progressive deepening or sink- 

 ing of the floor in the great oceanic basins is a view now 

 generally held, which, if well founded, accounts for the sub- 

 sidence of the Hawaiian ridge. 



The absence of drowned valleys and fjords, as well as the 

 great sea-cliffs where the waves have gnawed deep into the 

 peripheral volcanic deposits, speak against recent subsidence. 

 There is evidence of slight elevation in some places; but the 

 islands seem to have remained practically stationary since the 

 cessation of volcanic activity in the older masses. Geologists 

 are chary of expressing an opinion on the age of the volcanic 



