SOCIAL AND COMMUNAL LIFE 423 



ready to adapt itself to any new conditions of life. The 

 parasite has risked everything for the sake of a sure and 

 easy life under the present existing conditions. Change 

 of conditions means its extinction." 



For an elementary account of commensalism and para- 

 sitism see Jordan and Kellogg 's " Animal Life," pp. 172- 

 200. The account here given is based on the author's 

 previously written account in " Animal Life." See also 

 Van Beneden's " Animal Parasites and Messmates." 



