582 DISHARMONIES AND OTHER SHADOWS 



tsetse-fly belt are fatally infected by a trypanosome whicb 

 does not seem to damage the native antelopes in which it 

 is, so to speak, at home. The fatality of a new microbe 

 introduced into a new population is familiar, as in the case 

 of the Black Death in England, which was due to the in- 

 troduction of the microbe of bubonic plague. 



It must be remembered that the effect of the parasite on 

 the host is extraordinarily varied. Some give off toxic sub- 

 stances ; others cause lesions and inflanmiation, especially if 

 they stray from their usual habitat in the body; some pro- 

 mote beautiful growths like oak-apples and pearls; others 

 drain the food-supply ; some do very little harm. The sturdie- 

 worm causes locomotor disorders in the sheep in whose brain 

 it grows, but the Gregarines found in the reproductive or- 

 gans of most earthworms seem usually unimportant in their 

 effects. The parasitic crustacean known as Sacculina de- 

 stroys the reproductive organs of crabs and changes the 

 male constitution towards the female type, so that a small 

 ovary may develop. The shape of the crab's abdomen 

 changes, approximating to that of the female, and the pro- 

 truding parasite is actually guarded by its bearer as if it 

 were a bunch of eggs. But many ^ fish-lice ' seem to do very 

 little, if any, harm to their bearers. It is highly probable 

 that very aggressive parasites have eliminated themselves 

 from time to time by killing their hosts, which it is not to a 

 parasite's interest to do. 



(c) It seems useful to place by themselves parasites like 

 virulent Bacteria (e.g., the Plague Bacillus) and virulent 

 Protozoa (e.g., the Trypanosome of Sleeping Sickness) which 

 are rapidly fatal when transferred to a new kind of host. 

 Thus the Plague Bacillus is transferred by the rat-flea from 

 the rat, who can stand it, to man who has no constitutional 



