OP CONCHOLOGY. 203 



Dr. Gould writes: — 'The doctrine of local limitations meets 

 with so few apparent exceptions, that we admit it as an axiom 

 in zoology that species strongly resembling each other, de- 

 rived from widely diverse localities, especially if a continent 

 intervenes, and if no known or plausible means of communi- 

 cation can be assigned, should be assumed as different until 

 their identity can be proved. {Vide "Expl. Exped. Moll.Intr.," 

 p. xi.) Much study of living specimens must be made before 

 the apparent exceptions can be brought under the rule.' It 

 has, however, to be borne in mind that the researches of mo- 

 dern geology clearly point to considerable alterations in the 

 existing configuration of continents, and in the consequent 

 direction of ocean currents during the ascertained period of 

 many species now living. Nor are we warranted in the belief 

 that the existing fauna in any locality has been created at any 

 one time, or has radiated from any single spot. To study the 

 relations of living shells simply in connection with the exist- 

 ing map of the world must lead but to partial results. 



"It is interesting to observe, that, notwithstanding the proba- 

 ble connection of the oceans through the Eocky Mountains 

 during the Miocene age, there is extremely little similarity 

 between the special temperate faunas of Bast and West 

 America. Not a single species has yet been proved identical, 

 and the allied forms are but few in number. 



" When, however, we approach the region in which boreal 

 and sub-boreal forms occur, many species are found in common, 

 and between others there is but slight difference. Yet even 

 here there are more British than New England species in the 

 west-coast fauna. As might be expected, the British species 

 are, for the most part, those which are also found fossil, and, 

 therefore, have had time to diffuse themselves widely over the 

 hemisphere. It is, however, remarkable that many Crag species 

 have reached Eastern Asia and West America, which are not 

 found in Grand Manan and New England. It is also extra- 

 ordinary that certain special generic forms of the Crag re- 

 appear in the North Pacific. 



"The Vancouver and California districts have so many 

 characteristic species in common, (111 out of 492,) that they 

 must be regarded as constituting one fauna, differing as do 

 the British and Mediterranean regions. One fact must, how- 

 ever, be here specially noted, viz : the great peculiarity of the 

 island-fauna. Although the Sta. Barbara group are so near 

 the mainland, the dredge has not only produced many species 

 not known on the continent, but also many before considered 

 as essentially tropical. Along with these are not only some 

 species of types hitherto regarded as almost exclusively 

 Asiatic, but also some which belong to the sub-boreal district, 



