THE SIBERIAN MIGRATION. 229 



Continental. I should scarcely venture the attempt 

 to revive old memories and stir up again long 

 forgotten controversies, were it not for the fact that 

 many new points have arisen in the course of the 

 above inquiries, which appear to me so very difficult 

 to explain by the land-ice hypothesis, while they are 

 comparatively easy to understand when we adopt 

 the old theory of the marine origin of the boulder- 

 clay. But a few geologists even at the present day, 

 while believing in the land-ice theory, recognise that 

 the marine hypothesis should have some considera- 

 tion shown to it. I need only remind glacialists of the 

 work recently published by Professor Bonney. " The 

 singular mixture," he remarks (p. 280), "and apparent 

 crossing of the paths of boulders, as already stated, 

 are less difficult to explain on the hypothesis of 

 distribution by floating ice than on that of transport 

 by land-ice, because, in the former case, though the 

 drift of winds and currents would be generally in one 

 direction, both might be varied at particular seasons. 

 So far as concerns the distribution and thickness of 

 the glacial deposits, there is not much to choose 

 between either hypothesis; but on that of land-ice 

 it is extremely difficult to explain the intercalation of 

 perfectly stratified sands and gravels and of boulder- 

 clay, as well as the not infrequent signs of bedding 

 in the latter." 



Now with regard to the land -ice theory, several 

 serious difficulties present themselves in connection 

 with the origin of the European fauna. In the first 



