ECOLOGICAL SUCCESSION. 87 



occur "everywhere," although they are rarely found in moist 

 woods or in any situation in which they are not fully exposed to 

 the sun and may always live in similar conditions. 



There are great differences between open prairies and closed 

 forests. Shimek ('il) found that the evaporation in the un- 

 disturbed groves in eastern Iowa during July and August was 

 very much less than that in the prairies adjoining. From the 

 free surfaces of pans set in the ground so that the water which 

 they contained was level with the surface of the soil, the evapora- 

 tion of the groves was about 27 per cenc. of that of the prairie; 

 with cup evaporimeters about 37 per cent, and with Piche 

 evaporimeters about 47 per cenc. This is about the same as 

 the difference on Long Island between the inner side of Tran- 

 seau's salt marsh dominated by grass-like plants and his meso- 

 phytic forest. Sherff ('12) found the evaporation in a marsh 

 foresc to be a little less than that in the beech maple and from 1.8 

 to 2.6 times as great as in the lowest stratum of a marsh. 

 While differences and similarities of physical conditions are suf- 

 ficient to account for many peculiarities of ecological distribu- 

 tion, it must be recognized that the same species may occur under 

 different conditions and show difference in mores (Bohn, '09, 

 Allee, '12). 



5. Agreement of Plant and Animal Communities. 



Before discussing the problem of agreement between plant 

 and animal communities, ic is necessary to state what is meant 

 by agreement. According to present developments of the science 

 of ecology plant and animal communities may be said to be in full 

 agreement when the groivth form of each stratum of the plant com- 

 munity is correlated with the conditions selected by the animals of 

 that stratum. Questions of agreement are primarily questions 

 for experimental solution. Two types of disagreement are to 

 be expected. We may illustrate the first by a bog or marsh com- 

 munity. Considering plants rooted in the soil we note that 

 water is secured from the soil by the roots and is lost through 

 the leaves and twigs. Accordingly since bog soil is unfavorable, 

 due to the presence of toxins or to other causes, plants grow- 

 ing in it do not secure water easily even when the quantity 



