DISTRIBUTION. 397 



reciprocal dependences of the inferior on the superior. 

 Mr. Darwin's inquiries have shown how generally the 

 fertilization of plants is due to the agency of insects, and 

 how certain plants, being fertilizable only by insects of cer- 

 tain structures, are limited to regions inhabited by insects 

 of such structures. Conversely, the spread of organisms 

 is often bounded by the presence of particular organisms 

 beyond the bounds — either competing organisms or organisms 

 directly inimical. A plant fit for some territory adjacent to 

 its own, fails to overrun it because the territory is pre- 

 occupied b}^ some plant which is its superior, either in fertility 

 or power of resisting destructive agencies; or else fails 

 because there lives in the territory some mammal which 

 browses on its foliage or bird which devours nearly all its 

 seeds. Similarly, an area in which animals of a particular 

 species might thrive, is not colonized by them because they 

 are not fleet enough to escape some beast of prey inhabiting 

 this area, or because the area is infested by some insect 

 which destroys them, as the tsetse destroys the cattle in parts 

 of Africa. Yet another more special series of limita- 



tions accompanies parasitism. There are parasitic plants that 

 flourish only on trees of some few species, and others that 

 have particular animals for their habitats — as the fungus 

 which is fatal to the silk-worm, or that which so strangely 

 grows out of a New Zealand caterpillar. Of animal-parasites 

 various kinds lead lives involving specialities of distribution. 

 We have kinds which use other creatures for purposes of 

 locomotion, as the Chelonohia uses the turtle, and as a certain 

 Actinia uses the shell inhabited by a hermit-crab. We have 

 the parasitism in which one creature habitually accompanies 

 another to share its prey, like the annelid which takes up its 

 abode in a hermit-crab's shell, and snatches from the hermit- 

 crab the morsels of food it is eating. We 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. x\nd once more, we have the 



