DISTRIBUTION. 313 



insects. To these dependences of the relatively- superior 

 organisms on the relatively-inferior organisms which they 

 consume, must be added certain 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 a certain structure, are limited to regions 

 inhabited by insects of this structure. 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 that is fit for some 

 territory adjacent to its own, fails to overrun it, because the 

 territory is pre- occupied by some plant that is its superior, 

 either in fertility or power of resisting destructive agencies ; 

 or else 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 particu- 

 lar species might thrive, is not colonized by them, because 

 they are not fleet enough to escape some beast of prey inhab- 

 iting this area ; or because the area is infested by some in- 

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

 parts of Africa. Yet another more special series of 



limitations, accompanies parasitism. There are parasitic 

 plants that flourish only on trees of some few kinds; and 

 others that have certain 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- parasitism we have various kinds : severally involv- 

 ing their specialities of distribution. We have that kind in 

 which one creature uses another for purposes of locomotion ; 

 as the Clielonobia uses the turtle, and as a certain Ac- 

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

 that kind in which one creature habitually accompanies 

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

 its abode in the shell occupied by a hermit-crab, and snatches 

 from the hermit-crab, the morsels of food it is eating. Wo 



