STUDIES ON AVIAN HxEMOPEOTOZOA. 719 



those with which research Wiis principally occupied, the 

 possibility of such striking- polymorphism was insufficiently 

 recognised. It is evident, I think, that the safer plan for 

 workers on these naturally occurring trypaiiosomes will be to 

 regard all the forms met with in any one host as belonging 

 to one species until they have satisfied themselves that this is 

 not the case.^ 



On the other hand, foi- the purpose of distinguishing 

 different species of trypanosomes, I certainly continue to 

 think that what may be called the biological consideration 

 is, in the present state of our knowledge, the most reliable 

 and useful guide. By this I mean that the less closely related, 

 zoologically, two hosts ai-e, the greater the probability that 

 their trypanosomes are distinct species. As a general indica- 

 tion it may be said that the same parasite may, in certain 

 cases, be parasitic in different species of host, or even in 

 closely allied genera," but where the hosts in question belong 

 to different families, or still more, to different orders, it may 

 be safely assumed, as a working rule, that their trypanosomes 

 are distinct species. The best practical test for this criterion 

 is, of course, the production or non-production of cross- 

 infection. 



In making use of resemblances or differences in morphology 

 in compai'ing two trypanosomes, I think the ordinary adult 

 form of the parasite furnishes the best indications. Take the 

 case of T. lewisi, for example ; neither the young daughter- 

 individuals resulting from multiple fission, nor the large, 

 stout, multiplicative individual itself is regarded as the 

 definitive form, the form of every-day occurrence, as it were. 

 Now I think we can carry this comparison very usefully to 

 other cases. Small, fusiform, or stumpy individuals are more 



1 I consider, for instance, that Wenyon (37) has done wisely in includ- 

 ing the quite different types of form fovind, on the one hand, in the 

 gninea-fowl (Numida) and, on the other hand, in a lizard (Mabuia), 

 under one species in each case, viz. T. nnmid^ and T. mabuise. 



" In this connection attention must be paid to tlie qviestion of distri- 

 bution. 



