Z ENTOMOLOGICAL SOCIETY 



Association or symbiosis. Organisms may associate together 

 for different purposes and in different degrees of intimacy ; and 

 the association may be between different individuals of the same 

 (or very closely allied) species, or between individuals of different 

 species very widely separated from each other systematically. 



If two individuals, a male and a female of one species, associate 

 for the purpose of reproduction, we speak of "pairing" If 

 more than two individuals are concerned in this association (as, 

 for instance, in the case of bees, etc.) we speak of " colonizing.'" 



But as soon as the two contracting parties belong to natural 

 groups more or less widely separated (systematically) from each 

 other, we have before us some stage of parasitism. Although 

 this holds good as a general rule, the fact that the two individuals 

 belong to the same species does not necessarily exclude the asso 

 ciation -from the field of parasitism, for we find some remarkable 

 examples of "pairing" which have at the same time a certain 

 tinge of parasitism about them. For instance, we find in the 

 bladder of rats a species of Trichosoma (T. crassicauda) , in 

 which (Us Leuckart has shown) the males live in the uterus of 

 the female. Although the female may have a number of males 

 in her uterus, this symbiosis is unquestionably a case of pairing 

 (polyandrism) ; at the same time the symbiosis is a case of para 

 sitism more strictly speaking, a case of mutualism. 



At first thought we might suppose that the association between 

 individuals of the same species is a more common occurrence 

 than the symbiosis of organisms of widely different species ; in' 

 other words, that pairing is a more common occurrence than 

 parasitism, but we can convince ourselves that this idea is 

 erroneous if we recall that there is probably not one of us present 

 who has not, at some time during his life, harbored parasitic 

 worms, lice, and fleas; furthermore, that every one of us at the 

 present moment has a large number of species twenty or more 

 of bacteria in his mouth and upon his skin, and that of each 

 species we may harbor hundreds of thousands of individuals. ' If 

 we need further examples to convince us, we have only to ex 

 amine the first stomach of any ruminant, and we shall find there 

 countless hosts of bacteria and infusoria, belonging to numerous 

 different species. I would hence make the general statement 

 that parasitism in its different grades is a much more general 

 occurrence (among the higher animals, at least) than pairing ; or, 

 in other words, that organisms are associated with individuals of 

 other species more than with other individuals of their own 

 species. 



Parasitism. In parasitism, I would recognize several different 

 grades : first, mutualism, in which the svmbiosis results in 

 mutual benefit to the two contracting parties ; secondly, commen- 

 salism, in which the symbiosis results in a benefit to one party, 



