360 BOOKS AND CUERENT LITERATURE 



which' seedlings develop are largely due to the surrounding vegetation, 

 which by its size and density controls the evaporation beneath it. 



"The complete range of evaporation conditions present in this 

 region, namely, from bare ground to the mature forest, is completely 

 within the physiological limits of the seedlings of Acer saccharum, Pinus 

 strobus, Pinus resinosa and Thuja occidentalis. Given suitable soil 

 conditions, maple seedlings will develop under evaporation conditions 

 at least 2>'i1% more xerophytic than the normal hardwood forest, or 

 400% more xerophytic than the very dense forest. 



"Within their soil requirements and in the presence of light, the 

 establishment of the pine, beech-maple and Thuja bog associations is 

 independent of any particular conditions of evaporation. Conse- 

 quently a decrease in evaporation is not a prerequisite to succession. 



"The change in the rate of evaporation from the chamaephytic 

 layer is produced by the development in density of the invading vegeta- 

 tion. Being coincident with and not antecedent to it, the chavge in 

 evaporation is a result and not a cause of succession." 



At the beginning of his paper, Gates quotes the opinion of Gleason 

 and Gates, and the first statements of Fuller and We iver, as given 

 above, to show the discrepancy in the views on tliis subject. Fuller 

 (1914, p. 199) evidently feels this also, for he says that "It seems sur- 

 prising that Gleason has reached an opposite conclusion from somewhat 

 similar data." In spite of this, however, the later statements of Fuller, 

 as of Weaver, make it clear that they are thinking of evaporation not 

 as a separate factor but in its normal relation to water content, a general 

 principle with which the work and the conclusions of Gleason and Gates, 

 and of Gates are in full accord. It seems probable, however, that Fuller 

 and Weaver would emphasize the importance of evaporation in the 

 complex of water relations especially in a dry region such as the Palouse, 

 while Gleason and Gates evidently do not. It is clear that evaporation 

 is an indirect or remote cause of succession, since it affects the adequacy 

 of the water-content through transpiration as well as by loss from the 

 soil. The direct or primary causes must be sought in the available 

 water-content, which finally determines absorption and growth, and 

 in light, which controls the food supply, to say nothing of its effect 

 upon transpiration. 



All of the above investigations have brought us appreciably nearer 

 our goal in the study of succession. They all possess the distinct 

 merit of dealing quantitatively with reactions in their causal relation 

 to the serai sequence. They also serve to bring us closer to the realiza- 



