HEAT 



ir 



known to aflFect the distribution of animals 

 (Grinnell and Storer, 1924; Grinnell, Dixon 

 and Linsdale, 1930) : vegetation, food, rain, 

 humidity, soil moisture, pH of soil, tem- 

 perature, altitude, atmospheric density, 

 available breeding niches, available refuge 

 niches, light, cloudiness and competition. 



The distribution of animals in many parts 

 of the world is more or less closely tied up 

 with temperature. For instance, temperature 

 races of the fruit fly, Drosophila funebris, 

 exist in Europe. Northwestern populations 

 are more resistant to cold; southwestern 

 ones, to heat. All eastern populations, whe- 

 ther from the northeast or the southeast. 



ture of their respective habitats. Doubtless 

 their distribution is also affected by other 

 environmental factors. 



THE BIOCLIMATIC RULE 



In spring and early summer in temperate 

 latitudes, periodic phenomena, such as be- 

 ginning of blossoming for a given species, 

 ripening of fruit, or appearance of active in- 

 sects, usually come three or four days later 

 for each higher degree of latitude and for 

 each 100 to 130 meters of latitude from any 

 given base. In late summer and autumn, 

 similar relations can be recognized, but in 

 the reverse direction. In certain regions tem- 



Fig. 20. Approximate distribution of three "temperature races" of Drosophila funebris. (Re- 

 drawn from Timofeeff-Ressovsky, in Huxley.) 



show high tolerance to both heat and cold. 

 These differential resistances are correlated 

 with the respective temperatures of the re- 

 gions under consideration. Figure 20 shows 

 that the January isotherm of —5 degrees 

 runs from northern Norway to southeastern 

 Russia. The July isotherm of 20 degrees 

 runs from Lisbon on the Atlantic eastward 

 and then northward up to about 63 degrees 

 latitude in Russia. The spread between 

 these isotherms in the east encloses an area 

 with a seasonal diflFerence of 25 degrees 

 and reveals the continental climate of this 

 inland region. Thus, even a coarse analysis 

 of these temperature races of D. funebris 

 shows a high correlation with the tempera- 



perature relations vary also along a given 

 set of meridians. Thus Hopkins, who def- 

 initely formulated the bioclimatic relations 

 for the United States as a "law" of nature, 

 added that there was a seasonal retardation 

 of four days from west to east for each 5 

 degrees of longitude. This rule was origi- 

 nally worked out on the basis of observa 

 tions on North American and European 

 phenology. 



The speed of migration of birds gives a 

 convenient test for the application of this 

 rule to one prominent periodic phenomenon 

 in animal life. From New Orleans to south- 

 ern Minnesota, the average speed of migra- 

 tion for all species of birds is close to 23 



