HEAT 



109 



tions, soil temperature is nearly constant at 

 a depth of 2 meters, although the soil sur- 

 face may vary more than 56 degrees in one 

 day (p. 219). This state can be reached 

 much nearer the surface under less extreme 

 conditions, and in steadily shaded portions 

 of tropical rain forests, where even sun- 

 flecks are absent, temperatiire on the forest 

 floor may show little variation from one 

 year's end to another. Deeper parts of caves 

 characteristically have constant tempera- 

 tures. 



the Bay of Biscay is one example of such 

 a situation (Sverdrup, Johnson, and Flem- 

 ing, 1942). 



At the surface of the sea, diurnal tem- 

 perature changes average not more than 

 0.3° C; hence, short-Uved populations in 

 surface waters often live out their life cycle 

 without great temperature fluctuation. In 

 deeper waters, according to records taken 

 at fifteen stations in the Atlantic and Atlan- 

 tic-Antarctic oceans, the mean temperature 

 variation between depths of 4000 and 2000 



14° 



IQo 20° 22° 24° 26' 28° 3C° 32' 

 TEMPERATURE C° 



Fig. 17 

 reciprocal 

 from Kro£ 



. Duration of incubation of chrysalids of Tenebrio molitor in hours (T); V is its 

 and shows velocity of development; it approximates straight line (v). (Redrawn 

 ;h.) 



The bottom waters of lakes of the first 

 and second order (p. 95), whether arctic, 

 temperate, or tropical, do not vary much 

 from 4° C. at any season. Similarly, animals 

 that live in the ocean, the upper part of 

 the lighted zone alone excepted, meet little 

 if any temperature variation. Even well 

 within the Hghted region, at a depth of 100 

 meters in many temperate locations, annual 

 variations are on the order of 1° or 2° C; 



meters averaged only 1.3° C, with extreme 

 ranges from 0.3° to 2.3° C. At these depths 

 a great many generations must live under 

 temperature conditions that approach or 

 equal those we call constant in experimental 

 laboratories. We do not know what would 

 be the effect of subjecting animals from 

 such steady environments to the controlled 

 constant or variable conditions that charac- 

 terize studies in experimental ecology. This 



