360 THE INFLUENCE OF LIVING SUKEOUNDINGS. 



CHAPTER XII. 



THE SELECTIVE INFLUENCE OF LIVING ORGANISMS ON ANIMALS. 



In the foregoing chapter we have seen that two organisms com- 

 ing into physical contact may be able to exert a permanent 

 transforming influence over each other. But this purely 

 mechanical transforming or modifying process must always 

 have been preceded by selection ; for if all the larvje which 

 creep or swim on the earth or in the water were equally capa- 

 ble of settling on any plant or animal that accidentally came in 

 their way, these species would certainly be extirpated. Thus, in 

 order that such animals may continue to exist as ai-e capable of 

 affording shelter or food to a certain number of others, they 

 must be enabled to make a selection between the species which 

 crowd upon them as commensals or as parasites. This selection 

 may under some circumstances have been already effected bj'" 

 the other conditions of existence, as we saw in the first section ; 

 but a second process of selection may be performed on those 

 forms which have been able to outstep the limits thus imposed 

 upon them, by the animal they choose to settle upon. This is 

 of course always undesigned. A very striking example of this 

 selective power of individual animals on the larvre of parasites 

 is offered by the different forms of the family of the Bopyridas 

 among the Crustaceans. Many of the species, and particularly 

 those of the genus Bopyrus (see fig. 38), live in the branchial 

 cavities of crabs or of tailed Crustaceans, in which they always 

 produce an enlargement — sometimes a very considerable one — of 

 the branchial cavity. We may suppose that the young larvaj 

 pass into it with the current of water which enters the branchial 

 cavity close to the mouth to supply the gills with fresh water, 



