1'RELiMiNARY CONSIDERATIONS. 30 



the inefficiency of physical barriers, than is presented 

 by the almost total dissimilarity of the North 

 American Tropical and Sonoran Regions, though 

 in direct contact, contrasted with the great similarity 

 of the Boreal Regions of North America and Eurasia, 

 now separated by broad oceans, though formerly 

 united, doubtless, in the region of Behring Sea." 



To return to the composition of the European 

 fauna, we now know positively that a number of the 

 mammals and birds inhabiting Central and Eastern 

 Europe are of Siberian origin. How they came, and 

 when, will form the subject for discussion in Chapter 

 V. At present it will suffice to mention that in the 

 superficial deposits belonging to the Pleistocene 

 series of the North European plain have been dis- 

 covered the remains of many typical members of the 

 Siberian Steppe-fauna. Some of these, such as the 

 Saiga- Antelope (Saiga tartarica), Fig. 2, still inhabit 

 portions of Eastern Europe, whilst others have re- 

 treated to their native land. But it might be asked, 

 how is it known that these species did not originate 

 in Europe, and thence migrate to Siberia? Because 

 if they had originated on our continent, they would 

 have spread there. They would have invaded 

 Northern and Southern Europe, and they would 

 probably have left some remains in Spain, Italy, or 

 Greece. They would also have left some of their 

 relations in Europe ; but all their nearest allies, 

 too, are Asiatic. Moreover, and this completes, I 

 think, the proof of their Siberian origin, the Pleisto- 



