314 THE INDUCriOKS OF BIOLOGY. 



have again the commoner parasitism of the Epizoa, — animals 

 which attach themselves to the surfaces of other animals, and 

 feed on their juices or on their secretions. And once more, we 

 have the eqnallj' common parasitism of the Entozoa — creatures 

 which live within other creatures. Besides being restricted 

 in its distribution to the bodies of the organisms it infests, 

 each species of parasite has usually still narrower limitations : 

 in some cases the infested organisms furnish fit habitats for 

 the parasites only in certain regions ; and in other cases, only 

 when in certain constitutional states. There are 



various more indirect modes in which tlie distributions of 

 organisms affect each other. Plants of particular kinds are 

 eaten by animals, only in the absence of kinds that are 

 preferred to them ; and the prosperity of such plants, hence 

 partly depends on the presence of the preferred plants. Mr 

 Bates has pointed out that some South American butterflies, 

 thrive in regions where insectivorous birds would else destroy 

 them, because they closely resemble butterflies of another 

 genus which are disliked by those birds. And Mr Darwin 

 gives cases of dependence still more remote and involved. 



Such are the chief negative causes of distribution — the 

 inorganic and organic agencies, that set bounds to the spaces 

 which organisms of each species inhabit. Fully to under- 

 stand their actions, we must contemplate them as working 

 not separately, but in concert. We have to regard the 

 physical influences, varying from year to year, as now 

 producing an extension or restriction of the habitat in this 

 direction, and now in that ; and as producing secondary 

 extensions and restrictions, by their effects on other kinds of 

 organisms. We have to regard the distribution of each 

 organism, not only as affected by causes which favour multi- 

 plication of prey or of enemies within its own area ; but also 

 by causes which produce such results in neighbouring areas. 

 We have to conceive the forces by which the limit is 

 maintained, as including all meteorologic influences, iinited 



