142 THE MIGRATION OF BRITISH BIRDS 



dispersal. Species would not range north of any area 

 from which they were absent until conditions in that 

 area were less suitable, and therefore impossible for 

 southern extension towards it. The very conditions 

 that arc driving species north, or attracting an emigra- 

 tion movement, are in like manner prevailing in other 

 areas immediately east and west, and thus rendering 

 impossible a southern emigration, or what is better 

 described as a retrograde movement. 



One of the most intensely interesting facts bearing 

 upon this Law of Northern Extension is that of the 

 migration of birds in the valley of the Petchora. Messrs. 

 Seebohm and Harvie Brown, whilst stationed at Ust 

 Zylma in the spring of 1875, kept careful observation 

 on the migration of birds north along this river to the 

 North Russian tundras. They report {Roivleys Orn. 

 MiscelL, 1876, i. p. 245) that "there can be no doubt 

 that Ust Zylma lies somewhat out of the line of migra- 

 tion." These gentlemen give a list of no less than 13 

 species of birds that were all common summer migrants 

 to the tundra, yet did not j^ass this station on the great 

 river. The reason for this is obvious. In the first place, 

 we must keep in mind the fact that the present line of 

 Migration of a species follows the past line of Emigra- 

 tion or range extension of that species, and that this 

 emigration never takes place in the Northern Hemi- 

 sphere in a soutJierly direction [conf. p. 60). If we refer 

 to the map, we find that the Petchora, in \ery nearly 

 north latitude ^d"" , takes a sudden trend to the south- 

 west for nearly 150 miles, amounting in the aggregate 

 to as much as 50 miles south of north. Birds, then, in 

 emigrating into North Russia from the south-cast, and 



