72 OEIGIN OF LIFE IN AMEKICA 



It may be of interest now to inquire as to how far a group 

 of less mobile creatures than the mammals, the snails, for 

 instance, can b'e classified into species whose ancestors were 

 Asiatic, and such' as were originally native to the soil. Of 

 course we need only consider snails resident in the Atha- 

 baska— Mackenzie Region. A direct European influence 

 among the fresh-water molluscan fauna is apparent, since 

 certain species such as Limnaea stagnalis, L. truncatula, L. 

 palustris, and Aplexa hypnorum inhabit the Mackenzie 

 Region as well as Europe. How they have spread to America 

 is not readily ascertainable. The points of resemblance 

 between the two regions are clearly of long standing, but none 

 of these species need necessarily have come from Europe 

 direct, as they all inhabit Siberia as well as Canada. 



Among the land snails we also meet with forms familiar 

 to the European conchologist, such as Pupa muscorum, 

 Cochlicopa lubrica, Hyalinia radiatula, H. nitidula, Euconu- 

 lus fulvus, Zonitoides nitidus, and others. All these are no 

 doubt very ancient species, too ancient in fact to help us 

 materially in our present inquiries. It is possible, moreover, 

 that they have special facilities for accidental dispersal, that 

 is to say for dispersal other than the ordinary mode of pro- 

 gression on land, although my studies have not led me (to 

 believe in the efficacy of such a mode of conveyance in 

 permanently stocking a country. 



The strictly American Pupa armifera and P. holzingeri, 

 Vertigo ventricosa, V. ovata, Vitrina limpida, Patula solitaria, 

 and P. striatella are all easily transported by flooded streams, 

 and thus scattered far and wide. In spite of the fact that the 

 land and fresh-water mollusks of the Mackenzie Region 

 indicate that a distinct Old World influence is recognisable, 

 nothing points definitely to a recent land connection with 

 either Europe or| Asia. They do not confirm the view that any 

 survival of the molluscan fauna through the Glacial Epoch 

 has taken place in that region. On the contrary, the absence 

 of species peculiar to the region implies, as among the 

 mammals, that that part of America has only recently been 

 stocked with animals from another portion of the continent. 

 Whether we assume that gigantic glaciers covered the coun- 

 try, or whether we argue that the ocean invaded it, the 



