332 THE INFLUENCE OF LIVING SURROUNDINGS. 



tilising or depositing their eggs ; in spite of this they live just 

 as well as the individuals not attacked by parasites, and it may 

 be inferred from the size of their shells that they live equally 

 long. 118 



Now, it certainly is a fact that the influence exerted by one 

 living creature over another is in most cases selective only. A 

 transforming influence, on the other hand, occurs, and can occur, 

 only when the two species come into direct bodily contact. But 



FIG. 85. Part of the stem (<) of a hydroid Polyp, Campanula net, with closed pear-shaped 

 galls (6) within which lives the larva (a) of a sea-spider, Pycnoyonum. 



even then a selective influence is the first to come into play. 

 For example, if a certain species of hermit-crab were an equally 

 good host for the larvae of all the parasitic Crustaceans which 

 float or swim in the sea, it would in all probability soon be 

 wholly exterminated ; consequently the continued existence of 

 the parasites themselves depends not merely on their being able 

 to attach themselves to a suitable host, but also on the selection 

 which the host himself may be able to effect among the guests 



