202 SECRETS OF ANIMAL LIFE 



to that of the female, and the protruding parasite 

 is actually guarded by its bearer as if it were a 

 bunch of eggs. On the other hand, many external 

 parasites behave as if their end in life was to do for 

 their host what he will not do for himself, namely, 

 keep his skin clean. Great mortality from parasites 

 is in most cases due to immigrant animals entering 

 a fresh area and becoming liable to attack by para- 

 sites to which they can offer no natural resistance, 

 as when cattle enter the Tse-tse fly belt and become 

 infected with trypanosomes which are fatal to them, 

 though doing little or no damage to the indigenous 

 animals in which they are at home. Similarly, the 

 fatality of a new parasite in a new population is 

 familiar, as in the case of the Black Death in Eng- 

 land, which was due to the introduction of the 

 microbe of bubonic plague from the East. It is not 

 the parasite's interest to kill its host that is killing 

 the goose that lays the golden eggs but it is highly 

 probable that very aggressive parasites have elimi- 

 nated themselves from time to time by turning 

 into beasts of prey. For it seems almost legitimate 

 to place by themselves, and outside the ranks of 

 ordinary parasites, the very virulent microbes like 

 Plague Bacilli and Sleeping-Sickness Trypanosomes. 

 They are internal plants-of-prey and beasts-of-prey, 

 and it is interesting to notice that some of them 

 live an exceedingly active life, which is not the usual 

 habit of adult parasites. 



Many parasites are aesthetically repulsive in form, 

 color, and movements, and it is instructive to 



