CHAPTER LXII 



ASSOCIATION OF ORGANISMS SOCIAL BACKBONELESS 

 ANIMALS (INVERTEBRATA). 



Many animals are social or gregarious, and in such cases 

 division of labour between the members of a community is more 

 or less perfectly exemplified. The mere fact that many individuals 

 of the same species live together in the same place does not entitle 

 them to be termed social, in the sense here intended, unless there 

 is some sort of co-operation which benefits the animals living 

 together. It would hardly be justifiable, for example, to describe 

 oysters, cockles, or star-fishes, as colonial animals. But even here 

 the species may be benefited, e.g. weakly individuals are weeded 

 out in the keen struggle for existence, so that the stock becomes 

 increasingly healthy and strong. And from such casual kinds of 

 association communities of very complex kind have gradually 

 been evolved, the benefits to be derived from division of labour 

 between individuals giving various species a greatly improved 

 chance of survival in the competition with other species. But here 

 a qualification must be made. For in the world of organisms, by 

 the irony of fate, an extreme penalty attaches to elaborate special- 

 ization resulting from adjustment to the exigencies of a certain set 

 of conditions. The surroundings of animals (and plants) are con- 

 stantly changing, and if these alter suddenly, as they are liable to 

 do, a well adapted species may be unable to adjust itself with 

 sufficient rapidity to the new order of things, and hence be 

 doomed to extinction, while simpler but more plastic forms may 

 survive. Many lowly organisms have endured through countless 

 ages, while others of more complex kind have quickly succumbed 

 to rapid alterations of their environment at a time when their 

 continued dominance seemed most certain. Innumerable in- 

 stances of this far-reaching principle are to be drawn from the 

 geological record, which preserves for us the past history of the 

 globe. 



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