32 PLANT SUCCESSION AND CROP PRODUCTION 



ening the crevices. By the time a heath stage has been reached the 

 cliff is usually well covered. This stage is succeeded by a shrub stage 

 and often by a coniferous stage. Gradually oaks come in, increas- 

 ing the shade and lowering the water loss from the soil as well as 

 the oxidation rate of humus. The oaks are succeeded in time by 

 more mesophytic trees — beech and maple, which are again the 

 culmination of the successions. In all of this series the action of 

 the plants has been toward an increase in soil and moisture until 

 the conditions became suitable for plants like beech and maple. 



These successions teach the lesson of soil foimation and bring 

 out the importance of the biotic factor in soil building. They also 

 show the importance of the genetic conception and the inadequacy 

 of a static or even a merely dynamic system of classification, for 

 the plants become a cause of subsequent change. Successful inva- 

 sions are of course in the beginning the effect of certain conditions 

 of the bare area into which they migrate. Chance is the sole cri- 

 terion of the original seed or spore introduction, but the life of the 

 seedling after introduction depends upon its inherent ability to 

 meet the conditions of the environment. 



