ECOLOGICAL SUCCESSION. 75 



the effect of animals upon the soils in which they live but if 

 excretory products ever accumulate in any quantity, they 

 probably have a detrimental effect, especially upon the animals 

 which produce them (Colton '08 and citations). On the other 

 hand, many burrowing animals bury organic material and bring 

 mineral soil to the surface. The digger wasps must add much 

 to the sand by burying many insects for their young. Earth 

 worms appear in the later stages and contribute to soil formation 

 (Darwin). Cowles states further on the authority of Transeau 

 that humus accumulation alters soil aeration. 



(c) Temperature. Transeau found that the temperature of bog 

 soil and bog water is below that of other soils and waters. This 

 has however not been observed in the case of dry soils. The 

 differences between soil on the beach at Sawyer, Mich., Aug. 19, 

 1911, at 3.00 P.M. and in the beech woods near at hand was as 

 follows: Air 20 C., upper one half inch of sand of cottonwood 

 area 38-39 C., sandy soil of beech woods I9-2O C., a difference 

 of 19 C. The upper one half inch of bare sand goes as high as 

 47 C. on the hottest days of summer while the soil in the beech 

 woods is probably always a little cooler than the air at the time 

 of the air maximum. Cottonwood soil temperature on the hottest 

 summer days at about 3.00 P.M. has been found to be as follows: 



TABLE II. 



SHOWING VARIATION OF SAND TEMPERATURE WITH DEPTH AND MOISTURE CONTENT. 



AIR 36 C. 



Dry Sand. Moist Sand 



1.25 cm. below surface 47 C. 32 C. 



3-4 cm. below surface 38 C. 31 C. 



8-9 cm. below surface 35 C. 29 C. 



10-1 1 cm. below surface 33 C. 



12-13 cm. below surface 32 C. 27 C. 



17-18 cm. below surface 30 C. 



Simultaneous readings in later forest stages were impracticable. 

 Even where exposed to the sun moist sand is kept at a lower tem- 

 perature by the evaporation. 



3. Atmosphere. 



Conditions at and above the surface of the soil, i. e., in the 

 ground and field strata. 



