112 THE NAUTILUS. 



high or complicated nature, we ma}' suppose that not many centuries 

 would pass, after the Rocky, Sierra Nevada Mts. and the adjacent 

 territory rose above the " Mesozoic Sea," before these creatures 

 would originate, perhaps in many places at or about the same time; 

 and as they multiply in numbers quite rapidly, under favorable con- 

 ditions, there would have been a large native population of terrestrial 

 mollusks existing here long before those Asiatic stragglers could have 

 reached Cape Horn and the West Indies via Bering Straits bridge. 

 If the present land shells of America are the descendants of Asiatic 

 emigrants, what has become of the land shells that have originated 

 here ? 



" Westward the course of empire takes its way " is as true as it is 

 poetical, and if animals obey the same general laws that human be- 

 ings do in this respect, then the snail emigration must have been the 

 other way. We can as readily imagine bands of snails from America 

 crossing this bridge at Bering Strait and establishing colonies on 

 the other side as vice versa, and thus we could account for these re- 

 semblances and affinities by a westward movement as well as by an 

 eastward emigration of these slow-moving creatures, if there were no 

 causes or conditions in the environment in both countries to produce 

 similar results in the organic structure of the same class of animals, 

 which I believe is the case. 



Without having investigated the matter very closely, I am under 

 the impression that the resemblances and affinities of the West Coast 

 shells, as well as those of America generally, are as close to those 

 of Europe as they are to the shells of Asia. Compare the following 

 American and European shells : 



American. European. 



Helix nickliniana Lea. Helix arbustorum Linn. 



Helix kelletti Fbs. Helix aspersa Mull. 



Helix levis Pfr. Helix pisana Miill. 



Helix inflecta Say. Helix personata Drap. 



Patula striatella Anth. Patula ruderata Stud. 



Compare the entire Zonitidse of both continents. Many of Limnse- 

 idse, Physidse and Planorbidae on both sides of the Atlantic Ocean 

 are identical, or so near alike that they could hardly be separated if 

 mixed together. 



I think, then, we may reasonably conclude that if the emigration 



