5O HISTORY OF THE EUROPEAN FAUNA. 



to be confounded with our British Hyalinia (formerly 

 united with Zonites\ does not extend very far south 

 or north of the Alps. There may be others too, which 

 owe their origin to these mountains, but most of 

 the terrestrial mollusca are exceedingly ancient, and 

 many genera have existed long before the Alps had 

 made their appearance above the surface of the early 

 Tertiary seas. It should be remembered that Hya- 

 linia and Pupa, both British genera, are known from 

 carboniferous deposits in forms which closely approach 

 those living at the present day, and in these and a 

 great number of other instances, it is quite impossible 

 to determine the original home of the genus. 



This little digression on centres of dispersion will 

 help us to understand in what manner the indigenous 

 element of the European fauna joined in with the 

 alien members as they arrived in our continent. The 

 species confined to South-Eastern England need not 

 necessarily have come to us from Eastern Europe or 

 Siberia. Alpine species spread northward probably 

 at the same time as the Siberian animals went west- 

 ward. An Alpine form may therefore have joined a 

 batch of the latter and entered Eng-land with them. 

 Even a Lusitanian animal may have mingled with 

 these migrants, so that all three elements may occur 

 together in one locality. 



But these are exceptions. The migrations have, as 

 a rule, not joined to any great extent; indeed, all 

 those naturalists who have carefully examined the 

 problem of the origin of the European fauna, have felt 



