CHAPTER 5 



THE EFFECT OF PARASITISM ON 

 THE PARASITE 



Ruinous inheritance ! 



Gaius 



A parasite's life is an impressive gamble. Indeed it is difficult to 

 envisage insecurity on such a scale. The chances of a grouse round- 

 worm finding a grouse are far less than the reader's chances of becoming 

 the parent of quads, or a cabinet minister. 



Most free-living animals do not die of old age — they are killed and 

 eaten. The majority of parasites, on the other hand, die a lingering 

 death from hunger or exhaustion because they fail to find a host. As 

 Shipley pointed out, the eggs of the grouse roundworm lie scattered all 

 over Scotland, but millions and millions of their young, which hatch 

 out and wriggle up the sprigs of heather around them perish because 

 their particular plant is never eaten by a grouse. Similarly, vast 

 numbers of immature ticks cling hopefully to blades of grass, waiting 

 for the millionth chance which will bring an animal brushing through 

 the vegetation within reach of their waving forelegs. 



Owing to the difficulty of finding a host — a difficulty which is 

 superimposed on the more familiar hazards of life — the mortality 

 among most parasites is enormous. A vast number of eggs or larvae 

 have to be produced in order that the species can survive at all. Con- 

 sequently a characteristic feature of most parasites is a relatively 

 enormous development of the reproductive organs, which frequently 

 come to dominate the body. Intestinal worms produce eggs by the 

 million and even brood-parasites like the cuckoo lay four or five 

 times as many eggs as their hosts. The difficulty of host-finding can 

 often be estimated by the number of eggs laid. Female ticks of the 



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