826 REPORT — 1893. 



seek to generalise upon the distribution of animal or vegetatle life, how is it 

 possible to arrive at a mean geographical distribution of animals ? How many 

 genera of mollusks are equal to a genus of mammals, or how many butterflies are 

 equal to a bird ? 



If there be any region of the world with any claim to be a life area, it is that 

 part of the Polar Basin which lies between the July isotherm of 50° or 53° F. 

 and the northern limit of organic life. The former corresponds very nearly with 

 the northern limit of forest growth, and they comprise between them the barren 

 grounds of America and the tundras of Arctic Europe and Siberia. 



The fauna and flora of this circumpolar belt are practically homogeneous ; many 

 species of both plants and animals range throughout its whole extent. It consti- 

 tutes a circumpolar Arctic region, and cannot consistently be separated at Behring 

 Strait into two parts of suflicient importance to rank even as sub-regions. 



Animals recognise facts, and are governed by them in the extension of their 

 ranges ; they care little or nothing about generalisations. The mean temperature 

 of a province is a matter of indift'ereuce to some plants and to most animals. The 

 facts which govern their distribution are various, and vary according to the needs 

 of the plant or animal concerned. To a migratory bird the mean aunual temper- 

 ature is a matter of supreme indiflerence. To a resident bird the question is 

 equally beside the mark. The facts which govern the geographical distribution of 

 birds are the extremes of temperature, not the means. Arctic birds are nearly 

 all migratory. Their distribution during the breeding season depends primarily 

 on the temperature of July, which must range between 53° and 35° F. It is very 

 important, however, to remember that it is actual temperature that governs them, 

 not isotherms corrected to sea level. If an Arctic bird can find a correct isotherm 

 below the Arctic Circle by ascending to an elevation of 5,000 or 6,000 feet 

 above the level of the sea, it avails itself of the opportunity. Then the region of 

 the Dovrefeld above the limit of forest growth is the breeding plan of many abso- 

 lutely Arctic birds ; but this is not nearly so much the case on the Alps, because 

 the cold nights vary too much from the hot days to come within the range of the 

 birds' breeding grounds. Here, again, the mean daily temperature is of no import- 

 ance. It is the extreme of cold which is the most potent factor in this case, and 

 no extreme of heat can counterbalance its effect. 



_ In estimating the influence of elevation upon temperature it has been ascer- 

 tained that it is necessary to deduct about 3° F. for every thousand feet. The 



ISOTHEEMAL LIXES 



are very eccentric in the Polar Basin. The mean temperature of summer is quite 

 independent of that of winter. The isothermal lines of July are regulated by geo- 

 graphical causes which do not afl'ect those of December, or operate in a contrary 

 direction. The Gulf Stream raises the mean temperature of Iceland during winter 

 to the highest point which it reaches in the I'olar Basin, viz., 30° to 35° F., whilst 

 m summer it prevents it from rising above 45° and 50° F., a range of only 15°. In 

 the valley of the Lena in the same latitude the mean temperature of January is 55° 

 to 50° F. below zero, whilst that of July is C0° to 65° F. above zero, a range of 115°. 



The close proximity of the Pacific Ocean has a much less efiect on the mean 

 temperature at Behring Strait, which is in the same latitude as the north of Iceland. 

 The mean temperature for January is zero, whilst that for July is 40° F. The 

 mean temperature for January in the same latitude in the valley of the Mackenzie 

 IS 25° below zero, whilst that for July is 55° F. In this case the contrast of the 

 ranges is 40 and 80, which compared with 15 and 115 is small, but the geo- 

 graphicalconditions are not the same. Behring Sea is so protected by the Aleutian 

 chain of islands that very little of the warm current from Japan reaches the 

 straits. _ It is deflected southwards, so the Aleutian Islands form a better basis for 

 comparison. Their mean temperature for January is 35° F., whilst that for July 

 is 50° F., precisely the same difierence as that to be found in Iceland. 



The influence of geographical causes upon climate being at present so great, it 

 is easy to imagine that changes in the distribution of land and water may have 

 had an equally important influence upon the climate of the Polar Basin during the 



