THE SIBERIAN MIGRATION. 



207 



tendency as in the older ones of which we have fossil 

 evidence, viz., generally a spreading of species on a 

 large scale over new territory, and then a gradual 



FIG. 1 6. Map of Europe. The doited portions represent, approxi- 

 mately, the course of migration of the Siberian mammals. The 

 principal mountain ranges are roughly indicated in black. 



shrinkage towards their original home, with an occa- 

 sional survival of small colonies in the invaded part. 

 It must not be supposed that this observation applies 



