300 THE WONDER OF LIFE 



edulis and Elceagnus rhamnoides), and has proved its bene- 

 ficial nitrogen-fixing role. It appears that the microbes 

 are polymorphic, rod-Kke (bacillus) forms and spherical 

 (coccus) forms being found in the same plant. 



Parasitism. — ^When one organism fives in or on another 

 — ^its host, gets its food from it, is inextricably bound up 

 with it or with related forms, and is not beneficial but rather 

 injurious in its influence, we speak of parasitism. But, 

 as in other cases, the facts are too subtle for absolutely 

 precise definition. There are beautiful Infusorians in the 

 stomach of the horse, which are not found anywhere else ; 

 they apparently help rather than hinder the process of 

 digestion : Are they symbions or parasites ? Many small 

 Crustaceans are found on the sMn of fishes, where they 

 clean up mucus and the hke ; it is hard to draw the line 

 between some of them and the barnacles on a whale's 

 skin, which are merely epizoic. Not a few of the skin 

 parasites, e.g. mites, are doing their best to clean their 

 host. Many parasites seem to do no harm to their hosts 

 unless these get out of condition ; this is probably the case 

 with many of the threadworms and tapeworms found in 

 the food-canal of animals. Some parasites are quite 

 unimportant unless they get shifted into pecuhar situations, 

 such as the vermiform appendix in man, within which 

 Nematode worms often provoke inflammation, or unless 

 they become suddenly very numerous. It need hardly 

 be said that the definition of parasitism must be such as to 

 exclude the antenatal fife of the young mammal within 

 its mother, for here the two creatures are of the same 

 flesh and blood, and though the benefit is onesided and a 

 drain on the mother, she is adapted to her ofTspring as no 

 host ever is to its parasite. 



