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 sthetically repulsive in form, 
color, and movements, and it is instructive to 
