106 EVOLUTION 



worms, tapeworms and other parasites, are 

 not, in many cases at least, mere modification 

 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 unaccustomed host ; as probably 

 in the case of sleeping sickness. 



Again, parasitism may pass on one side 

 towards more and more complete mutual 

 adaptation, witness the symbiosis of alga 

 and animal in certain sea-anemones, or 

 the admirable permanence of that co-opera- 

 tion of short-lived alga and transient mould 

 which enables the resultant lichen sometimes 

 to outlive the very tree which bears it. Galls, 

 again, afford many instances of a parasitism 

 which is reaching equilibration. 



Thus in many ways we must not consider 



