NOTES ON ZOOGEOGRAPHY. 



brought there by drifting logs, or sticking to the feet or 

 feathers of birds. 



We have to account for the presence of representatives of 

 eight families of land and three of fresh-water snails. 1 There 

 must have been at least sixteen, probably more, original 

 immigrants. The affinities of practically all are with Poly- 

 nesia, and much more remotely with the western border of 

 the Pacific, in no case with America. Under present condi- 

 tions, drift material can come only from America. This has 

 certainly been the case during the duration of the present 

 islands, as their coast topography shows. In all probability 

 the trades have blown and the current flowed as they do now, 

 since early geologic times. 



The idea that the mollusks were carried by wind, as some 

 insects and seeds may have been, is hardly to be entertained.. 

 The distance is too great. 



The hypothesis that the Hawaiian volcanoes rise from 

 a preexisting mid-Pacific ridge, now lost by subsidence,, 

 gives room in time and space for the development of the 

 peculiar fauna. The tertiary history of the islands, on this 

 hypothesis, has been mainly one of subsidence along with 

 volcanic upbuilding. This subsidence has continued down to 

 comparatively recent times. Such topographic forms as Pearl 

 Harbor, Kaneohc Bay, Kahana Bay, can hardly be explained 

 except as valleys of subaerial erosion. On the northern coast 

 of Kahoolawe it may be noticed that the ravines and ridges 

 extend out into the sea in a succession of little bays and points, 

 although the present erosive action tends to wear away the 

 points and silt up the bays. 



There is also, as many authors have noted, conclusive evi- 

 dence that on Oahu there has been a small Pleistocene eleva- 

 tion. This certainly amounted to 20 ft., as beds containing 

 recent marine shells indicate. According to Professor Hitch- 

 cock, the bluffs at Kahuku are of marine origin up to 60 ft., 



1 The following groups, now well established, are regarded as introduced 

 in the human period : Limacidce, PhilomytidcB, Helicidce, Opeas, Gcecilioides, 

 Planorbis, Viviparus and probably Ancylus and Musculium. 



