38 GEOGRAPHICAL DISTRIBUTION 



breediug-rauge 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 further north 

 in Scandinavia than it does in East Russia, and in most cases where the 

 breeding-range extends across Asia the birds breed further 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 repre- 

 sents midsummer in the Arctic Regions, rise much further 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 24< 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 10 degrees of latitude would make a difference of 20 days in 

 the duration of summer — a very important curtailment of the time neces- 

 sary for the successful rearing of a brood where the summers are so short. 

 The fact that snow disappears 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, harmonizes 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. 



It must, however, be remembered that the mean temperature is only an 

 approximate index to the climate of a district. The mean temperature in 

 July of the Dovre Fjeld is the same as that of the plains three or four 

 hundred miles further north. The shortness of the summer nights in 

 latitude 62° causes the climatic condition of the two localities to vary so 

 little that their fauna and flora are almost the same. It is obvious that 

 somewhere in the Alps a similar mean temperature is to be found in July ; 

 but in latitude 47° the climatic conditions are very different : the tem- 

 perature during the day is considerably raised by a much more vertical 

 sun, and the much longer duration of night causes the extremes of 

 temperature to vary so much during the twenty-four hours that a very 

 different climate is the result. Adult birds easily adapt themselves to 

 considerable extremes of temperature provided always that they can 

 obtain a sufficient supply of food ; but most of them are cai'eful to avoid 



