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 
