158 H. M. WOODCOCK. 



itself iiud the hitler. The parasite, on its part^ enjoys, at 

 first, a more Insty aucl vigorous developmeut than usual in 

 the new and fertile soil, rapidly gains an ascendancy and 

 overcomes its host, which, sooner or later, almost invariably 

 succumbs. 



Thus it happens that the discovery of these minute and in- 

 conspicuous blood parasites has usually been the result of an 

 endeavour to ascertain the cause of various perplexing mala- 

 dies (malaria, trypanosomosis, piroplasmosis, etc.) to which 

 " civilised " man and " imported " animals in these regions 

 are subject. It follows, however, from what has been said 

 above, that the animals for which these parasites are markedly 

 pathogenic cannot be regarded as their true or natural hosts, 

 Avhich are rather to be sought among the native, tolerant 

 animals of the locality concerned. 



Abnormal and Involution Forms. — This method of 

 discovery, moreover, tends, unfortunately, to militate against 

 a thorough investigation of the morphology and life-cycle of 

 the parasites themselves. The medical authorities by whom, 

 in most of these cases, new disease-causing parasites have 

 been first discovered were, as of course was only natural, 

 chiefly intent upon an investigation of the malady and its 

 prevention and cure. The jDarasite was studied, more or 

 less incidentally and from a pathological point of view, 

 attention being focussed principally upon the relation of its 

 different forms to the different phases of the illness, etc. 

 But when the reaction between the attacked organism and 

 the invader is particularly severe (as in trypanosomosis, for 

 example) great care is required in determining the exact rela- 

 tioTi in which any given form of the parasite stands to the 

 typical life-history of the same. Especially is this so in the 

 case of a Trypanosomc, for here, at any rate, the parasite by 

 no means has matters all its own way. A strenuous fight for 

 life is made by the host's cells and tissues in Avhich many of 

 the organisms, notwithstanding their remarkable vitality, 

 certainly suffer. As Laverau and Mesnil, two prominent 

 researchers on this group, have pointed out, there can be no 



