Xlvi NOTES ON ZOOGEOGRAPHY. 



cerning the growth and history of the islands, especially as 

 to their relative age, and the idea that they were built up 

 from the sea bottom solely from materials ejected by the 

 volcanoes. This hypothesis presumes an enormous output 

 and a duration very long for the active life of a volcano. 

 Nowhere on land, where the geologic structure is accessible, 

 has any mass even remotely comparable in magnitude to the 

 Hawaiian ridge been piled up by purely volcanic agency. 

 While it is true that most volcanoes rise from low levels, it 

 is also noteworthy that those whose summits stand high 

 (18,000 to 30,000 feet above the general level of the 

 supporting sea bottom, in the case of the Hawaiian 

 ridge) are borne on the backs of great folded mountain 

 uplifts, as the Andes, or the Mexican plateau. Since 

 none of the known volcanic deposits are believed to be older 

 than tertiary the hypothesis also limits the time available for 

 the evolution of the peculiar Hawaiian fauna to a period far 

 shorter than experience has shown is probable. Elsewhere 

 we know that many groups of generic rank in the land shells 

 go back to the Oligocene and some to the Eocene. The dif- 

 ferentiation of the modern families must therefore have taken 

 place largely in the Mesozoic. It is not likely that so strongly 

 characterized a family as the Achatinellidce is much later. 

 The case would be different if Achatinellidce were known to 

 have existed on any of the continents, for then it might be 

 claimed that they had drifted to the islands. Personally, I 

 think the drift hypothesis, applied to these remote islands, is 

 about as credible as the idea that life was first brought to 

 earth on a meteorite. I can conceive that wood-boring beetles 

 might travel in the logs from America which I have seen 

 thrown upon the windward coast of Molokai logs stripped 

 of bark and worn deep into the solid wood by the buffeting 

 of the waves but my faith is too weak to believe that any 

 snail could get deep enough in the wood to make the journey 

 of two thousand miles dry, or that if it did, it would ever 

 get out when the log finally dried on a tropical beach. Only 

 arm-chair zoogeographers can hold to the hypothesis that the 

 land snails, Lymn&idce and Melaniidce of these islands were 



