52 REPORT ON THE MIGRATION OF BIRDS. 



of immigrants, week by week and month by month, to the 

 eastern shores of these islands, coming directly across Europe 

 from E. to W., or more commonly from points S. of E. to others 

 N. of W., and the reverse in the spring. These are mainly 

 composed of those common and well-known species which 

 annually make these islands their winter resort, and take the 

 place of our summer birds : they come in one broad stream, 

 cutting the line of ordinary migration at nearly right angles ; 

 one flank brushes the Orkneys and Shetlands, pouring through 

 the Pentland Firth, even touching the distant Faroes ; the 

 southern wing crosses the Channel Islands, shaping its course 

 in a north-westerly direction to the English coast. In our 

 explanation of the causes which first induced, and perhaps still 

 influences, this E. to W. migration, we must probably go back a 

 long way in the history of the world, when the distribution of the 

 land and water of continental Europe was very different to what 

 it now is ; when there was no North Sea, and the western coast- 

 line of Europe was represented by what is now known as the 

 hundred -fathom line off' the West of Ireland, a coast which on 

 the one side touched Scandinavia, and on the other was linked 

 with the Spanish peninsula. Great as is now the contrast 

 between the winters of Central Russia and those of these 

 islands, the difference would then be much more marked, — 

 arctic cold on one hand, and semi-tropical warmth on the 

 other.* It requires then no stretch of imagination to believe 

 that great flights of birds would on the approach of winter be 

 driven before the intense cold of Eastern and Central Europe to 

 seek refuge and find food in the warm regions of the west, 

 regions which then would feel the full effects of the warm 

 equatorial currents, and enjoy an almost perpetual summer. 

 This movement once begun would, by the very necessities of 

 existence, and in time by an hereditary instinct, be continued. 



* There are ninety species of j^lants, all told, common alike to South- 

 western England and Ireland, and to the Pyreuean and ItaUan region. 

 They represent an old flora no longer adapted to the country, — a flora of 

 warmth and sunshine, — and now dying out under the advance of hardier, 

 more vigorous and congenial species. They may be regarded as the last 

 floral relics of the submerged land, that semitropical western land whose 

 plants and flowers are not of Scandinavian origin, but derived from Southern 

 Europe. 



