CHAPTER XVIII 



ASSOCIATION OF ORGANISMS 



Nature of Association. — The number of individuals 

 composing a species remains practically the same from 

 century to century. There may be a marked increase 

 when conditions are unusually favorable, and again there 

 may be lean years when there is an equally sharp decline, 

 but over a long period of years the average is fairly uni- 

 form. Furthermore, the number of young produced 

 by the members of the species is greatly in excess of those 

 arriving at maturity (see Chaps. XXVI and XXXV). 

 Some of the young are driven into unfavorable 

 situations, others are destroyed by enemies, and 

 always there is a fierce struggle for food where 

 only a comparative few emerge as conquerors. In 

 this combat the individuals of most species of 

 different kinds of animals are wholly self-reliant, depend- 

 ing upon their own strength and keenness of sense to 

 escape destruction. But while there are many species 

 where " life is a continuous free fight, " it can not be 

 denied that among higher animals especially there are 

 associations of individuals where the strong protect the 

 weak and display a spirit of cooperation and mutual aid 

 akin to that in man. Again, there are many kinds of 

 animals and plants in which one individual enters into 

 mutually advantageous relations with another animal or 

 plant of a totally different type. And, finally, there are 

 many species which in the keen struggle for life 

 have taken up positions on or in the bodies of other 

 organisms from which they derive their nourishment. 

 The association of organisms is therefore a broad, com- 



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