CHAPTER I 



PARASITISM 



Almost 

 All the wise world is little else, in Nature, 

 But parasites and sub-parasites. 



Ben Jonson 



THE WORD parasite means, literally, one which eats beside another, 

 but the modern biologist cannot accept this as a definition. It is too 

 elastic and too vague. The term is now generally used to indicate 

 strict dependence between two organisms, one of which, at any rate, 

 during some period of its life, lives at the expense of the other. The 

 word parasite is often used in a broader sense to mean any animal or 

 plant which is dependent upon a host — and it is left to the reader to 

 decide for himself how exactly to define the term host. 



Ultimately all animals depend on plants or other animals for their 

 source of energy. They must eat to live. Plants may subsist on an 

 ethereal diet — largely air and water flavoured with sunshine — but 

 animals require more substantial if less romantic fare. 



Many biologists see in a parasite a form of predatory animal. 

 Instead of killing and devouring its prey whole, it can, by virtue of its 

 smaller size, live on the host or in it, and eat it little by little. The robin, 

 for example, has a number of relatively large carnivorous enemies such 

 as hawks, cats, rats and stoats which prey upon it and devour it whole. 

 It also supports a far greater number of animals smaller than itself 

 (Fig. i) which are parasitic and live by gradually eating relatively 

 minute portions of its body. Elton has described the difference between 

 a carnivorous and a parasitic mode of life simply as the diflference 

 between living on capital and income. If, however, an animal becomes 

 a parasite, the problems which confront it and the consequences of its 

 mode of life are unquestionably dififerent from those of a free-living 



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