Soil-Group Divisions 89 



thus seen to be an outstanding example of a system formed by the 

 interaction of organism and environment. 



The review of the substratum as an ecological factor given in this 

 chapter has revealed the great variety of surfaces and solid materials 

 on and in which animals and plants live. The manner in which the 

 nature of the substratum controls the distribution and the growth of 

 different species has been indicated. We have also stressed the fact 

 that the activities of living organisms— and the material of their bodies 

 after death— may profoundly alter the substratum in both the aquatic 

 and the terrestrial environments. Sometimes this process itself brings 

 about the further progressive changes in the fauna and flora known as 

 ecological succession, considered in Chapter 12. Most of all, our 

 present discussion focuses attention upon the reciprocally dependent 

 relation existing between the living community and the substratum. 

 The nature of the mud on the bottom of a pond or of the soil on the 

 surface of the land is partly the result of the biological influences and 

 is at the same time partly responsible for their existence. 



The physical conditions of the water or the land originally allow 

 certain plants and animals to exist in a given area. These living 

 agents may then modify the substratum, whereupon the substratum 

 may be further affected by the "climatic" factors, perhaps resulting 

 in additional changes in the fauna and flora. When we visit the area 

 after these activities have gone on for a long time, and are still going 

 on, it is not easy to distinguish cause and effect. Nevertheless, with 

 a good grasp of the relations summarized in this chapter we are better 

 equipped to determine the specific action of climatic, physiographic, 

 and biological influences in bringing about the existing conditions. 



