THE STRUGGLE FOR EXISTENCE 423 



that every species of animal and plant is fitted to a 

 definite set of conditions, and that within its range each 

 species finds a limited food supply, enemies of many 

 kinds, and unfavorable as well as favorable conditions of 

 soil and climate. All of these agencies tend to hold the 

 number of individuals at a given level. 



The Struggle for Existence. — It is also manifest 

 that since many more individuals are produced than can 

 come to maturity, a struggle for existence is the inevitable 

 result. Broadly speaking, this is a battle against the sur- 

 rounding conditions, an effort, conscious or otherwise, on 

 the part of the organism to place itself in the most favor- 

 able relations to the environment. In this combat ani- 

 mals play a more obvious part, but it is none the less true 

 that plants are at war with one another, and victory 

 comes to a very few. This struggle against unfavorable 

 conditions presents a threefold aspect. The individuals 

 of the same species may compete with one another. A 

 plant may become overshadowed by its fellows and perish 

 for lack of sunlight. A rat may fight with its own kind 

 and even devour its young. Or members of one species 

 may enter into competition with those of another. The 

 eagle strikes down a fawn, and locusts consume the har- 

 vest crops. Finally, there is a continual striving on the 

 part of an organism to withstand extremes of heat and 

 cold, a lack or a superabundance of moisture, and other 

 unfavorable physical features of its surroundings. 



Interrelations of Organisms. — The environment of 

 an animal or a plant is more than a simple mixture of 

 light, heat, air, moisture, and soil, plus other organisms. 

 These elements are parts of a complex system where- 

 in a relatively small modification at one point may 

 produce highly important changes in a distant and fre- 

 quently unexpected quarter. In this. Nature resembles 

 a pool where a pebble dropped into the depths causes 

 far-reaching effects. 



Darwin calls attention to the fact that bumble bees 



