CHAPTER 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 i 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, 
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