THE PROBLEM OF PARASITISM 201 



The repugnance which many people feel when 

 they think of parasites is partly practical. They re- 

 sent the fact that a contemptible microbe kills the 

 genius before he comes of age, and that paltry flies 

 put a drag on the wheel of the chariot of civiliza- 

 tion. But this is a one-sided view. Many parasites 

 do little harm to their host; a modus vivendi has 

 been established. The thousands of Nematodes 

 in the food-canal of a grouse seem of no moment 

 if the bird be healthy. If it be of a weakly consti- 

 tution, however, the parasites, otherwise trivial, 

 may gain the upper hand and eliminate their host. 

 As this sifting makes for racial health it cannot be 

 called abhorrent. The effects of parasites on their 

 hosts are extraordinarily varied; some give off 

 toxic substances; others, like the beautiful Infu- 

 sorians in a horse's stomach, appear to be to some 

 extent helpful; some cause internal lesions, and 

 others provoke beautiful imprisoning growths like 

 the oak-apples in the wood and the pearls in the 

 oyster. The sturdie-worm causes locomotor ataxia 

 in the sheep whose brain it inhabits, but fish-lice 

 seem often entirely unimportant to their bearers. 

 Almost every earthworm has parasitic Gregarines 

 in its reproductive organs, but they are not usually 

 of moment; on the other hand, the parasitic 

 Crustaceans known as Rhizocephala actually destroy 

 the reproductive organs of crabs. More than that, 

 they change the constitution of the male towards 

 the female type, so that a small ovary sometimes 

 develops; the shape of the abdomen approximates 



