OF BRITISH BIRDS ,>, , IOI 



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 farther 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 

 temperature 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 careful to avoid great extremes for 

 their young. Hence, though we find an epitome of Arctic orni- 

 thology on the tundras of the Dovre Fjeld, we look in vain for a 

 similar epitome on the higher Alps. Though the mean tempera- 

 ture during July may be the same, the cold nights produce a 

 climate sufficiently different to prevent most Arctic Birds from 

 using it as a breeding-ground. It cannot be too clearly laid down 

 that the latitudinal distribution of Birds is purely climatic. 



The division of the summer climate of the world into three 

 zones -Arctic, Temperate, and Tropic is scarcely narrow enough 

 for the present purpose, and it will be more convenient to restrict 

 somewhat the limits of the Arctic and Tropic zones, and split the 

 Temperate zone in two parts, which might be called Subarctic and 

 Subtropic respectively. 



The mean temperature for July would then range as follows 

 in each of the four climates : 



Arctic from 40 to 53, including Iceland, the Highlands of 

 Scotland, North Scandinavia, the Dovre Fjeld, the tundras of East 

 Russia and Siberia, and the mountains of East Siberia. 



Subarctic from 53 to 65, including the British Islands, Central 

 Europe, South Siberia, and the north island of Japan. 



Subtropic from 65 to 77, including South Europe, the moun- 

 tains of North Africa, Asia Minor, Central Asia, North China, and 

 the south island of Japan. 



Tropic from 77 to 90, including the plains of North Africa, 

 Arabia, India, South China, and the Malay Archipelago. 



