104 EVOLUTION 



of parasites are among the very strangest 

 disentanglements of the web of life, but into 

 the stories of these adventures in search of 

 mischief we cannot enter here: enough if we 

 note the stupendous rate of multiplication 

 by which the many chances against finding 

 the proper host are constantly met; thus 

 the common tape-worm of man has been 

 calculated to produce eighty-five million ova 

 during its two years' existence. 



It is an interesting inquiry whether the 

 large numbers of so-called species of thread- 

 worms, tape-worms and other parasites, are 

 not, in many cases at least, mere modifica- 

 tion forms, whose diagnostic characters are 

 directly induced by the peculiarities of 

 their respective hosts. The question is, of 

 course, one for the experimental observer. 



Again, even parasitism must not be viewed 

 too pessimistically. It is, after all, not the 

 interest of the parasite to kill its host, or 

 even to deteriorate its life too seriously; 

 moreover the host becomes more or less 

 adapted to its wonted guests, and probably 

 correspondingly immune to the irritant 

 poisons which many parasites have been 

 shown to excrete. The rapid disaster which 

 parasites so often bring about seems rather 

 when introduced into some new and unac- 

 customed host; as probably in the case of 

 sleeping sickness. 



