CHAPTER XIX 



PARASITISM 



Ix the preceding chapters I have drawn tlie reader's 

 attention to tlie fact tliat the characteristic feature 

 of the principal tropical diseases was, that they w^ere 

 caused by animal parasites, many of wliich were so 

 complex that they required two hosts for their com- 

 plete development ; for example, the parasite of 

 malaria, and, in all probabihty, tliat of yellow fever. 

 Dependent upon this remarkable peculiarity, I pointed 

 out that tlie fundamental principle in prophylaxis was 

 the destruction of one of the hosts, namely, the 

 insignificant insect host. That only by such means 

 could we be absolutely certain of stamping out the 

 disease in question. In this chapter I wish to still 

 further emphasise this point, and to introduce the 

 reader to some of the other problems raised by the 

 study of parasitism. 



In the first jilace, we now begin to realise how 

 novel land epoch-making were the discoveries of 

 Pasteur and his school, which revealed to us the 

 struggle for existence continually taking place in 



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