IOO GEOGRAPHICAL DISTRIBUTION 



high latitude, or by a high elevation at a comparatively low 

 latitude. For example : the climate of the Dovre Fjeld, 6000 

 feet above the level of the sea, in latitude 62, is very similar to 

 that of Lapland, 60 feet above the level of the sea, in latitude 70, 

 and many purely arctic birds and plants are found in both 

 localities. 



The other factor which appears to play an important part in 

 deciding the breeding range of many birds, especially of Passerine 

 Birds, which require a long time to rear their young, is the length 

 of the summer. 



The breeding range of many British birds extends much farther 

 north in Scandinavia than it does in East Kussia, and in most 

 cases where the breeding range extends across Asia the birds breed 

 farther north in West Siberia than they do in the extreme east of 

 that country. The cause of this peculiarity in the distribution of 

 these birds can scarcely be ascribed to difference of temperature. 

 The isothermal lines of July, which represent midsummer in the 

 Arctic Regions, rise much farther north between the White Sea 

 and the Delta of the Lena than they do in Scandinavia or 

 Kamtschatka. The Atlantic cyclones in the west cause the 

 summers of Scandinavia to be cold and wet, whilst the winter 

 accumulation of ice in the Sea of Okotsk lowers the summer 

 temperature of Kamtschatka and the east coast of Siberia. 



The probable cause is the difference in the duration of the 

 summers in the different districts. In any given longitude the 

 summers are shorter as the latitude increases. In the valley 

 of the Yenesei I found that the summer advanced at the rate 

 of 100 miles in twenty-four hours, which may represent a degree 

 a day after due allowance has been made for the winding of the 

 river. Assuming that the advance of winter is at the same rate, 

 a difference of ten degrees of latitude would make a difference 

 of twenty days in the duration of summer a very important 

 curtailment of the time necessary for the successful rearing of a 

 brood where the summers are so short. The fact that snow dis- 

 appears in North Scandinavia much earlier than it does in the 

 same latitude east of the White Sea, whilst it lingers much 

 longer in the extreme east of Siberia than it does in the same 

 latitude in Western Siberia, harmonises with the variation in the 

 latitude at which many birds range during the breeding season, 

 so that it is difficult to avoid coming to the conclusion that they 

 are cause and effect. 



