THE EVOLUTION OP ENVIRONMENT 59 



less of origin, as soon as life arrived on earth it immediately began 

 to alter its home, even as a house that is lived in reflects the charac- 

 teristics of the residents. In general the lower orders of living 1 

 things tended to improve their habitat (from man's point of view) 

 in the sea and on land. At least they changed it in such a way that 

 other life forms could exist. 



The primitive plants which attached themselves to rocks or rock 

 materials began to exert both physical and chemical forces on the 

 earth, changing it. The green plants stored sun energy and on 

 dying added it to the emerging soil. This started the accumulation 

 of energy in the land which eventually became the reservoir of 

 power and fertility serving the human race. The presence of green 

 plants both living and dead made possible the existence of fungi, 

 which further modified, complicated, and improved the physical and 

 chemical properties of this very thin earth layer. 



When rooted plants appeared they penetrated the soil accumula- 

 tions (and even porous, cracked or cleft rocks) loosening, irrigating 

 and aerating. The roots of dead plants remained at various depths, 

 adding their elements and energy to the soil mantle. As the roots 

 decayed and shrank in size the channels they occupied were invaded 

 by acidified water. This water had become acid by combining with 

 the carbon dioxide of the air and that produced by plant (and later 

 by animal) cells. This acid solution, as noted previously, served as a 

 soil manufacturing agent. Rooted plants also collected minerals 

 from the earth and carried them upward, depositing them, when 

 death came, on or near the surface, thus building fertile topsoil. 

 (Opposed to this was leaching, in which water carried soluble min- 

 erals deeper. Which effect was greater depended on the amount of 

 rainfall, temperature, and the porosity of the soil.) 



Green plants released surplus oxygen, altering the atmosphere. 

 Pood and oxygen being available, animal life appeared, living on 

 and in the soil, or in the waters where soil elements had accumulated 

 through land drainage. These animals formed and released carbon 

 dioxide into the air (or water), which was used by still greater plant 

 populations. During the entire process here described the environ- 

 ment was being enriched in such a fashion that more and more or- 

 ganisms could live in it. This cumulative effect went on and on. 

 Organic matter which had wrested minerals from the soil and put 

 them into first class condition for use by life, returned them to the 

 soil on dying, where they were quickly and easily used by new gen- 

 erations. The hard work was done: The process had become not 

 only physical and chemical, but biological as well. Rich topsoil was 

 fabricated by life and death. 



By this time, a mature soil, instead of being plain "dirt," or a 

 simple mixture of chemical elements in the form of physical granules 

 or aggregates (clumps), had become an extremely complex, dynamic 



2 Soils and Man, U. S. Department of Agriculture, Washington, D. C., 1938, p. 

 887. 



