CHAPTEK X 



COMMENSALISM AND SYMBIOSIS 



90. Association between animals of different species. The 



living together and mutual help discussed in the last chap- 

 ter concerned in each instance a single species of animal. 

 All the various members of a pack of wolves or of a com- 

 munity of ants are individuals of the same species. But 

 there are many instances of an association of individuals 

 of different kinds of animals. The number of individuals 

 concerned, however, is usually but two that is, one of 

 each of the two kinds of animals. In many cases of an 

 association of individuals of different species one kind 

 derives great benefit and the other suffers more or less 

 injury from the association. One kind lives at the expense 

 of the other. This association is called parasitism, and is 

 discussed in the next chapter. In some cases, however, 

 neither kind of animal suffers from the presence of the 

 other. The two live together in harmony and presumably 

 to their mutual advantage. In some cases this mutual 

 advantage is obvious. This kind of association is called 

 commensalism or symbiosis. The term commensalism may 

 be used to denote a condition where the two animals are 

 not so intimately associated nor derive such obvious mu- 

 tual advantage from the association, as in that condition 

 of very intimate and permanent association with obvious 

 co-operative and marked advantage that may be called 

 symbiosis. A few examples of each of these interesting 

 conditions of association between which it is impossible to 

 make any sharp distinction, will be given. 

 172 



