Obstacles to Life 337 



The Struggle for Existence 



The process which has been figuratively described as the 

 struggle for existence is a necessary part of our concept of 

 living things. As already indicated (pages 147 ff ) , the mainte- 

 nance of protoplasm involves constant adjustments and re- 

 adjustments to the environment, which in turn is a constantly 

 changing medium. The need for air, for example, involves 

 more and more complex arrangements in the evolution of 

 life. Although oxygen itself is plentiful, the increase in the 

 total volume of living matter makes substantial inroads into 

 the supply. Energy-consuming protoplasm could not main- 

 tain itself over a long period unless at the same time the 

 supply of oxygen were replenished, as it is by the starch- 

 building processes of green plants. Plants and animals do 

 not manifest any obvious effort in breathing, and we there- 

 fore do not think of the struggle for air as part of the living 

 process. When we consider food, however, the " struggle " 

 is more apparent, since only a few of the very simplest or- 

 ganisms and those containing leaf green are capable of build- 

 ing up food material out of inorganic substances. 



All species of animals and many species of plants are 

 dependent for their existence upon the vegetation. To say 

 that a given area can maintain a certain number of animals 

 is to say that the plants in the territory are able to produce, 

 in addition to what they need for their own maintenance, 

 a surplus in the form of leaves, roots and seeds to feed these 

 animals. Eventually all the energies of living beings on 

 the earth are traced to the sunlight, since it is by means of 

 sunlight that organisms containing leaf green are able to 

 build out of raw inorganic substances the food essential to 

 the maintenance of life. The struggle of the grass is for 

 water and salts from the soil and of carbon dioxid and sun- 

 light from above. Each individual seed is in conflict with 

 hundreds of others in the effort to establish for itself a place 

 in the sun and a foothold in the soil. We must remember 

 that Darwin used the expression " struggle for existence " 



