242 HISTORY OF THE EUROPEAN FAUNA. 



SUMMARY OF CHAPTER V. 



What has been spoken of in the earlier parts of this book as the 

 eastern migration, refers in a general way to the animals which 

 have come to England from the east. But these are by no means 

 natives of one country alone. We can trace a number of the 

 British mammals to a Siberian origin, and also some birds; 

 among many of the lower vertebrates and invertebrates, how- 

 ever, there are few species which have reached us from Siberia. 

 They may have had their original homes in the Alps, in Eastern 

 Europe, or in Central and Southern Asia, and have joined in 

 their westward course the later, more quickly travelling 

 mammals. Many instances are given from all the more im- 

 portant groups of animals to show how we may proceed in 

 approximately identifying the home of a species. 



The periodical invasion into our continent of Pallas's Sand- 

 grouse and other birds, suggests an explanation as to the cause 

 of the great westward migration in former times of the Siberian 

 mammals. Since a considerable amount of fossil evidence is 

 available to show the path of migration pursued by these 

 mammals, other important problems, such as the time of their 

 arrival in Europe and the geographical conditions surrounding 

 them, may perhaps be approximately ascertained, and thus 

 throw much light on the general features of the European 

 fauna. It has been proved by Professor Nehring that the 

 Siberian mammals arrived in Eastern Europe after the deposi- 

 tion of the lower continental boulder-clay. He believes 

 that the climate of Germany at that time had ameliorated so 

 far, after the maximum cold of the Glacial period, that steppes 

 with a Siberian fauna could exist. Other groups, such as the 

 Mollusca, however, do not support Professor Nehring's theory, 

 and in order to arrive at an independent solution of this and the 

 other problems referred to, a short history is given of the Siberian 



