662 The Outline of Science 



and is not beneficial but rather injurious in its influence, we speak 

 of parasitism. But a clear-cut definition is impossible; many 

 parasites do very little harm to their hosts unless these get out of 

 condition; many parasites are unimportant unless they get into 

 some vulnerable part of the body; many external parasites clean 

 up their host's skin. A parasite may become a symbion, as has 

 probably occurred in the case of the heather-fungus. There are 

 many degrees of parasitism, from external hangers-on like fleas 

 and lice to internal boarders like tapeworms and threadworms. 

 There is also an important distinction between parasites that live 

 in the food-canal of a host, sharing in the digested food, as tape- 

 worms do, and those that prey upon the living tissues like the liver- 

 fluke, which feeds on the blood of the sheep's liver. It is not in 

 the interest of parasites to kill their host; that is lopping off the 

 branch on which they are seated. Perhaps the trypanosome which 

 causes sleeping-sickness in man is not so much a parasite as a 

 predacious Infusorian devouring man from within as a lion 

 might do from without. A very important consideration is 

 that a parasite often establishes a live-and-let-live compro- 

 mise with its host, and nothing remarkable happens unless the 

 parasite is transferred to a new host which is not in any 

 way accustomed to such an intruder. Thus transference to 

 man is, as it were, an accidental episode in the life-cycle of the 

 trypanosome of sleeping-sickness not that the life-cycle is as 

 yet clear. 



It is important to see parasitism in its proper perspective: 

 it is a way of evading part of the struggle for existence. Just as 

 some animals have discovered caves, others have discovered hosts. 

 The discovery usually means abundant food and safe shelter; it 

 often involves a very riskful life-history. It is an interesting fact 

 that in some types, e.g., among crustaceans and insects, only the 

 females are parasites, which suggests that the habit sometimes 

 arose in connection with egg-laying and the protection of the 

 offspring. 



