ECOLOGICAL FACTORS 381 



inhabiting form. North American moles of the genus Scapanus (and 

 less clearly Scalops) are quite black in regions of abundant precipita- 

 tion (west Washington and Oregon) and become lighter with greater 

 aridity; thus in northern California they are brown, in southern Cali- 

 fornia silvery. 12 The forest lemming inhabiting the damp pine forests 

 is darker than its relatives, and similar color gradations have been 

 observed in the serval. 13 



A heavier rainfall may decidedly affect the faunistic character- 

 istics. The fact that butterflies are not found in Iceland, while they are 

 present in very much colder lands, may be explained by the rainy 

 summer of this island which interferes with flight and therefore with 

 copulation of the butterflies. 14 



Temperature. — The evaporating power of the air may be the 

 primary factor upon which the organization of the entire terrestrial 

 fauna depends. Once protection against desiccation has been acquired, 

 however, the effect of temperature upon the distribution of land ani- 

 mals appears to be as powerful and usually more evident. Although 

 the 20-inch isohyet, which approximates the hundredth meridian of 

 west longitude in the United States, is a well-marked line of distribu- 

 tion which does not coincide with physiographic features of the land- 

 scape, 15 the temperature boundaries that limit the distribution of an 

 animal species are many and much more distinct than the variations 

 in humidity. Humboldt states that the curves of the isotherms, espe- 

 cially of equal winter temperatures (isocheims), manifest themselves 

 in the boundaries that certain plants and animals seldom cross in the 

 direction of the poles, or in the direction of the peaks of snow-covered 

 mountains. The elk (Alee alces), for example, lives almost 10° farther 

 north on the Scandinavian peninsula than in the interior of Siberia, 

 where the line of equal winter temperatures becomes so strikingly 

 concave. 16 The northern boundary of the regular range of the migra- 

 tory locust Pachytylus migratorius coincides with the June isotherm 

 of 20°C., 17 and the southern boundary of the Arctic butterfly, Colias 

 -palaeno follows the January isotherm of —1° or —2°. 



Land animals in their turn may be stenothermal or eurythermal, 

 and the stenothermal animals may be limited to high, low, or inter- 

 mediate temperatures. 



All classes of terrestrial animals are represented among the steno- 

 thermal warmth-limited animals, which live primarily in the tropics. 

 In the temperate zones, such animals are confined to exceptionally 

 warm places, to warm islands or to protected southern slopes. In 

 Germany the region around Freiburg is a focus of animals with Medi- 



