INTRODUCTION. Xlll 



masses, in the total absence of palseontologic evidence. Those 

 best acquainted with aqueous erosion as displayed in the 

 Grand Canyon, etc., hold the opinion that the furrowing of 

 the older Hawaiian volcanoes may readily have been accom- 

 plished in Neocene time, and possibly since the beginning of 

 the Pliocene. 



In this stationary period of the islands, during which the 

 present complex topography has been developed from simple, 

 unfurrowed slopes, the specific and varietal evolution of the 

 Achatinellidte seems largely to have taken place. The topo- 

 graphic features more or less closely coincide with or define 

 the ranges of species and varieties. The development of 

 topography and the evolution of species and varieties evi- 

 dently proceeded pari passu. In former times, before the 

 slopes became so rugged and the contrasts of ridge and valley 

 conditions so emphatic, species doubtless spread much more 

 freely than they do under present conditions. Thus in 

 Hawaii Amastra flavescens has spread from the Hamakua 

 district down to the southeastern slope of Mauna Loa. With 

 subsequent isolation the more plastic of these widely-spread 

 forms have evolved into chains of allied species or races. 

 Precisely similar phenomena have been recorded by the author 

 from the mountain chains of southern Arizona, where the 

 bed of a canyon may separate distinct but related species. 



Dana was the first to show that the Hawaiian volcanoes are 

 progressively newer toward the southeast. Button qualifies 

 this generalization : in the northwest they became inactive 

 longer ago, and have therefore been sculptured by erosive 

 activities for a longer time. In fact, Kauai, western Oahu, 

 West Maui and northwestern Hawaii are long-extinct vol- 

 canic masses; eastern Oahu and East Maui newer, and the 

 rest of Hawaii still in building. So far as is known, the 

 Kohala region in Hawaii may be as old as Kauai. 



Dana's generalization, which is thus subject to consider- 

 able qualification, was doubtless the basis of Professor Hyatt's 

 hypothesis that the snails migrated from island to island, 

 from Kauai southeastward. A considerable acquaintance with 

 land shells causes me to doubt whether snails of moderate 



