THE INTEGRATION OF THE ORGANIC WORLD. 403 



their decay. Of higher types of epiphytes which use trees 

 only to gain elevation, the orchids may be instanced. And 

 then we have plants which, like the mistletoe, fix themselves 

 on the bark of their hosts, utilizing them partly for purposes 

 of elevation and partly by appropriation of their juices. After 

 these may be named those extreme cases in which the para- 

 sitic plants, ceasing to have any chlorophyll-bearing leaves, 

 live wholly on the juices of the invaded plants. At home 

 the common dodder, and in the tropics the Rafflesiacece, 

 belong to this group. There must be added the numerous 

 forms of minute fungi which in like manner thrive at the 

 expense of the plants they infest. In all these cases the 

 interdependence is one-sided, though, as we shall presently 

 see, while detrimental to one of the two concerned, it is not 

 always detrimental to the organic world as a whole. 



That utilization of one by another among animals which 

 causes immediate death, is familiar enough in the relations 

 between carnivores and herbivores. Almost as familiar are 

 those seen in parasitism. Less familiar are those seen in 

 commensalism ; and the least familiar are those which show 

 us exchange of services. Among these last the mutually- 

 beneficial relations that between the crocodile and the bird 

 which picks parasites out of its teeth is a striking one; and 

 no less so is that of the pique-gouffe, an African bird which 

 pierces the tumour on a buffalo's back that incloses a para- 

 site. Then of another kind we have the connexion between 

 aphides and ants : the one profiting by being carried to 

 better pastures and the other by increased saccharine excre- 

 tion. Next comes the class of messmates, the connexions 

 between some of which are relatively innocent, as witness 

 the Sea-anemone which settles itself on the shell occupied 

 by a Hermit-crab, or as witness the Remora fixed on a 

 shark's skin. Less innocent is the relation under which one 

 of the two seizes a share of the food obtained by the other, 

 like the annelid which insinuates itself between the Hermit- 

 crab and the whelk-shell it inhabits, or like the small fishes 



