GENERAL ZOOLOGICAL FACTS AND THEORIES 297 



says, " natural selection, orthogenesis, and mutation appear to 

 present fundamental contradictions; but I believe that each stands 

 for truth, and reconciliation is not distant " (280). 



2. THE SOCIAL LIFE OF ANIMALS 



Very few animals lead a solitary life, and even those that may 

 be considered solitary are influenced indirectly by others, just 

 as cats, by destroying field mice, determine the number of 

 bumblebees in a given locality (see p. 262). The various grades 

 of partnership and cooperation are recognized by the terms 

 commensalism, symbiosis, parasitism, and social life. 



a. Commensalism 



This is a term used to indicate a loose association of two kinds of 

 organisms from which one may derive benefit at the expense of the 

 other. In some cases one species eats at the table of another 

 without contributing anything in return. The guest bumble- 

 bee, that frequents the nests of the true bumblebees and lives 

 upon the food stored there, belongs to this class. The barnacles 

 which attach themselves to the skin of whales derive benefit 

 from being carried about, and, although they do not seriously 

 inconvenience their bearer, they fail to pay their fare. 



b. Symbiosis 



In symbiosis the association between the two kinds of organisms 

 is intimate and persistent, often showing marked cooperation and 

 being mutually advantageous. This kind of partnership may 

 have had its origin in the more simple commensalism, or may have 

 arisen from parasitism. Several illustrations of symbiotic rela- 

 tions have already been discussed in this book, that between two 

 plants (the lichen, p. 143), that between a plant and an animal 

 (Hydra and the green alga, Zoochlorella, p. 143), and that between 

 two animals (the hermit crab and a Ccelenterate, p. 143). 



