THE SIBERIAN MIGRATION. 215 



ice-sheet, no agency existed in North Siberia which 

 was able to suppress and to annihilate the forest and 

 meadow vegetation, and drive away the fauna con- 

 nected with it. We know, continues Tcherski, that 

 such genera as Bison, Colus (Saiga\ Rhinoceros, 

 Elephas, and Equus are met with in all horizons 

 of the diluvium of West Siberia. He therefore 

 comes to the conclusion (p. 474), that these and 

 other facts imply that the retreat of the North 

 Asiatic fauna commenced about the end of the 

 Tertiary Era (Pliocene), and that it was continued 

 very slowly throughout the Post-tertiary (Pleistocene) 

 Epoch, without any visible changes in its southward 

 direction, even during the time of the most important 

 glacial developments in Northern Europe. Only after 

 the conditions disappeared which had produced the 

 augmentation of an atmospheric moisture, did the 

 climate of North Siberia become deadly to a 

 temperate fauna and flora. Tundras then spread 

 over the meadow-lands and remnants of forests, 

 whilst arctic animals replaced the large ungulates 

 and carnivores which had wandered far away from 

 their native southern home. 



This is Tcherski's explanation of the extraordinary 

 events which he has chronicled, after years of the 

 most arduous labour and under conditions of peculiar 

 hardship. And though his work cannot be over- 

 estimated, and his opinions should receive the most 

 careful consideration, yet I fear the explanation will 

 not be looked upon as entirely satisfactory. Every one 



