298 NATURAL SCIENCE. Oct., 



Basin which lies between the July isotherm of 50 deg. or 53 deg. 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 

 is practically homogeneous ; many species of both plants and animals 

 range throughout its whole extent. It constitutes a circumpolar 

 Arctic region, and cannot consistently be separated at Behring Strait 

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

 The mean temperature of a province is a matter of indifference 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 annual 

 temperature is a matter of supreme indifference. 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 deg. and 

 35 deg. 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 ft. above the 

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

 the Dovrefield above the limit of forest growth is the breeding place 

 of many absolutely 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 importance. It is the 

 extreme of cold which is the most potent factor in this case, and no 

 extreme of heat can counterbalance its effect." 



The Field Naturalist. 



Canon Tristram's address to the Biological Section, as might 

 be expected, is devoted to the interests of the Field Naturalist. It 

 is a matter of gratification, for once, to turn from the modern 

 professor and his laboratory to a distinguished biologist whose work 

 has been chiefly accomplished in the field. Observations of organisms 

 in their natural surroundings are not merely essential for the under- 

 standing of such broad questions as geographical distribution, 

 variation, mimicry, migration, and so forth ; but they are also, as the 

 Canon points out, often indispensable even in researches for which 

 museum cabinets are commonly deemed to furnish adequate material. 



"The closet systematist is very apt to overlook or to take no 

 account of habits, voice, modification, and other features of life which 

 have an important bearing on the modification of species. To take 

 one instance, the short-toed lark [Calandrella brachydactyla) is spread 



