THE H-^MOPLAGELLATES. 289 



itself vary almost as mucli at different times and under 

 different conditions (see under Morphology), For the present, 

 at any rate, a very useful aid towards distiuguishing dif- 

 ferent species is fui-nislied by the biological relations of the 

 parasites. Speaking generally, it may be assumed that here, 

 as is known to be usually the case in the Sporozoa, a particular 

 species is restricted either to one particular host or, at most, 

 to a few closely allied ones. The greatest difficulty arises in 

 considering the Mammalian forms, many of which have never 

 been observed in the true natural hosts, but only in various 

 " foreign " animals, for which they are all more or less patho- 

 genic. The immunisation experiments of Laveran and Mesnil 

 (see above, p. 176) render it most likely, however, that such 

 forms are, at all events, distinct varieties, and, quite probably, 

 distinct species. 



On the other hand, having regard to the gross or extreme 

 differences in form which are met with (cf. T. rotatorium 

 and T. inopinatum) one might be tempted to arrange the 

 different species into sub-genera, grouping together, for 

 example, parasites with a blunt or rounded anterior extremity, 

 or those with a filiform or attenuated one, and, again, 

 separating forms with no free prolongation of the flagellum 

 posteriorly, from those which have one. Bearing in mind, 

 however, the great polymorphism which is known to occur, any 

 such arrangement would be quite arbitrary and certainly pre- 

 mature in the existing state of our knowledge. It may very 

 well be that when more life-histories come to be revealed some 

 of the forms at present included in the genus Trypanosoma 

 will have to be transferred to one of the other genera of 

 Haemoflagellates or placed in a new one. 



In this ai'ticle the writer considers it best to arrange the 

 different species under the different classes of Vertebrate 

 hosts in which they are parasitic ; this will, at any rate, 

 facilitate reference to any particular form. 



