156 H. M. WOODCOCK. 



the matter from the point of view of the parasite, it may 

 not be advantageous for this to cause, by its ravages, the 

 functional disorganisation or })remature deatli of its host. 

 For one grouj) of pai-asites especially would such a procedure 

 be likely to have disastrous consequences, namely, the Ha^ina- 

 tozoa, which are dependent for the completion of their life- 

 cycle upon being able to pass into an alternate Invertebrate 

 host at the moment when it sucks the blood of the Verte- 

 brate. In this case, therefore, we may say that such 

 mutual toleration ^ exists between the parasite and its host, 

 as, in ordinary circumstances, enables a proper balance to be 

 maintained on both sides. This equilibrium is disturbed only 

 w^hen the situation is affected by adverse influences (e. g. an 

 unusually strong infection, or weakness of the host owing to 

 unfavourable seasonal or nutritive conditions, etc.). 



Pathogenicity of Trypanosoraes. — These considera- 

 tions may afford some explanation of the non-occurrence of 

 trypauosomosis,^ or illness due to a Trypanosome, under 

 normal conditions in nature. It is a very different matter 

 when animals and parasites belonging to distinct regions are 

 brought into contiguity to one another. A Hasmatozoaii, 

 and especially a Trypanosome, produces marked pathogenic 

 effects upon gaining an entry into organisms which, previously, 

 have never been, by their distribution, liable to its invasion. 

 As Lankester (I.e.) points out, such a state of affairs is con- 



' It is, pcrliaps, desirable to empliasise the fact that Hasmaiozoa, whetlier 

 we regard Hajmosporidia or Trypanosomes, do not, in natural conditions, 

 cause any serious injury to their hosts. Consider, for instance, the Haemo- 

 sporea and the Trypanosomes of cold-blooded Vertebrates ; and, again, tlie 

 very great number of Avian hosts in wliicli malarial ])arasites appear to be 

 more or less generally present (cf. tlie observations ol: Galli-Valerio [23], 

 Sergent [102], and others). 



- This term is adopted in preference to various others in use for the 

 following reasons: (o) it has the right of priority, having been proposed 

 by Brumpt in 1901; (b) it agrees with the nomenclature of all other 

 Protozoan diseases, e. g. coccidiosis, piroplasmosis, etc. ; and (f) it is ])ure 

 and not a latinized hybrid, like trypanosomiasis, for example (see lilanchard, 

 ' Arch. Parasit.,' viii, p. 572). 



