398 THE INDUCTIONS OF BIOLOGY. 



equally common parasitism of the Entozoa creatures which 

 live within other creatures. Besides being restricted to the 

 bodies of the organisms it infests, each species has usually 

 still narrower limits of distribution; in some cases the in- 

 fested organisms furnish fit habitats for the parasites only 

 in certain regions, and in other cases only when in certain 

 constitutional states. There are more indirect modes 



in which the distributions of organisms affect one another. 

 Plants of some kinds are eaten by animals only in the absence 

 of kinds that are preferred to them ; and hence the prosperity 

 of such plants partly depends on the presence of the preferred 

 plants. Mr. Bates has shown that some South American 

 butterflies thrive in regions where insectivorous birds would 

 destroy them, did they not closely resemble butterflies of 

 another genus which are disliked by those birds. And Mr. 

 Darwin gives cases of dependence still more remote and in- 

 volved. 



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 re- 

 strictions by their effects on other kinds of organisms. We 

 have to regard the distribution of each species as affected 

 not only by causes which favour multiplication of prey or of 

 enemies within its own area, but also by causes which pro- 

 duce such results in neighbouring areas. We have to conceive 

 the forces by which the limit is maintained, as including all 

 meteorologic influences, united with the influences, direct 

 or remote, of numerous co-existing species. 



One general truth, indicated by sundry of the above illus- 

 trations, calls for special notice the truth that all kinds of 

 organisms intrude on one another's spheres of existence. Of 



