THE ART OF PARASITISM 



217 



Some Host-Parasite Relationships 



In the event of parasitism, the association is definitely in favor of 

 the parasite, since it usually "lives on" the second party concerned, 

 the host. Such a relation- 

 ship constitutes a fourth 

 type of habitat, namely 

 parasitic, that is available 

 to both plants and ani- 

 mals along with the well- 

 recognized terrestrial, 

 fresh-water, and marine 

 habitats. That many or- 

 ganisms take advantage 

 of this type of existence 

 may be clearly proved by 

 observing the plants and 

 animals of any locality. 



The Host-Parasite 

 Conflict 



Theoretically a conflict 

 exists between the para- 

 site and the host. The 

 latter has as its chief 

 weapon a lytic or dissolv- 

 ing power which is a nor- 

 mal physiological reaction. Likewise the weapons that probably 

 were first brought by the parasite from its hypothetical free living 

 ancestral state must also have been of a lytic, toxic, or otherwise 

 destructive nature. In many cases the host seems to have adapted 

 itself to bear the burden of parasitism with the least possible 

 outlay of energy on its own part, so that eventually there has devel- 

 oped a balance between the two organisms, which might be called 

 a Jwst-parasitc equilibrium. In order to reach this equilibrium the 

 parasite has likewise gradually e^'olved some sort of protectixe device, 

 often a capsule which becomes interpolated in the cycle, or an anti- 

 enzyme or anticoaguUn to counteract the destructive action of the 

 host's secretions, thus necessitating a counter attack upon the part 

 of the host. This apparently was made, first, through the o\or- 



W right I'icTCe 



The large masses in this tree represent a true 

 plant parasite, the mistletoe. 



