I04 THE MIGRATION OF BRITISH BIRDS 



(^Island Life, second edition, p. 339): "Germany, for 

 example, possesses nearly 90 species of land mam- 

 malia, and even Scandinavia about 60, while Britain 

 has only 40, and Ireland only 22. The depth of 

 the Irish Sea being somewhat greater than that of 

 the German Ocean, the connecting land would there 

 probably be of small extent and of less duration, thus 

 offering an additional barrier to migration [emigra- 

 tion], whence has arisen the comparative zoological 

 poverty of Ireland." The Reptiles furnish even more 

 significant evidence, owing to their more limited 

 powers of dispersal. Belgium has some 22 species 

 of reptiles and amphibia, Britain but 13, and Ireland 

 only 4 ! Again, if seas were not such an important 

 check to Emigration, we may very naturally ask why 

 no continental species attempt to colonize the British 

 Islands during historic time ; why do none of the birds 

 breeding so commonly almost within sight of our woods 

 and pastures ever seek to extend their area to us ? So 

 far as I am aware there is no evidence whatever to 

 suggest that any one species of bird has extended its 

 range to the British Islands since their final severance 

 from continental land at the Strait of Dover ; but on 

 the other hand we have ample proof that many species 

 have increased their range within our limits even during 

 historic time, as we shall learn in a future chapter {conf. 

 p. 168). It follows, then, that our present avifauna was 

 established here during land connection with Europe ; 

 not merely by the Strait of Dover, but during such 

 time that the English Channel formed one unbroken 

 land surface with the north of France. Even within 

 our area we have further confirmation of this interesting 



