CHAPTER VI 

 THE EVOLUTION OF ENVIRONMENT 



Breaking up the totality of life and environment into fragments 

 has disadvantages. Yet such fragmentation is necessary if proper 

 attention is to be given to the individual factors in the complex. The 

 burden of holding the total picture in mind is placed upon the reader. 

 We shall keep that burden as light as possible, but in doing so a 

 certain amount of repetition of phrases and terms cannot be avoided. 

 Thus, in discussing briefly the evolution of environment we cannot 

 avoid bringing in the role of plants and animals because they are a 

 part of it and contribute to it. The only exception to this is the 

 period before life appeared on earth. 



The Home of Life. The mineral earth is the foundation of envi- 

 ronment; but no more so than water and air. Of the earth itself, 

 the rock mantle or crust, a variable layer ranging from nothing up 

 to hundreds of feet of thickness, is the most important. Of this 

 crust, the first few inches of the surface (the topsoil) have come to 

 be the dispensing agent for the mineral salts essential to life (Fig. 

 18). The study of these phases of earth science is of course the field 

 of the geologist. Yet he cannot explain his field without calling on 

 other scientists the astronomer, for instance. 



The astronomer reports 1 that the distance of the earth from the 

 sun provides a temperature whose degree and range permit life as 

 we know it. Too little distance would mean more heat and the 

 vaporization of all water. Too much distance would bring perma- 

 nent ice. In either of these conditions it is difficult to conceive of 

 either plant or animal bodies, since they are composed largely of 

 liquid water. The size of the earth is also favorable for life. The 

 atmosphere is conveniently adjusted by gravitation, providing a 

 density and pressure in which the carbon-oxygen and other gas 

 exchanges can take place in organisms. A smaller planet has not 

 the gravity to hold such an atmosphere. If man ever colonizes the 

 moon, let us say for the purpose of mining uranium or other min- 

 erals which he had exhausted on earth, he will be forced to take 

 v/ith him or there manufacture a suitable atmosphere. A planet 

 larger than the earth has atmosphere of such great density as to 

 block insolation (absorption of sun heat at the planet's surface). 



The astronomical conditions involving the earth are responsible 



iHenderson, Lawrence J., The Fitness of the Environment, Macmillan Co., 

 New York, 1913. 



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