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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