244 THE HAEMOFLAGELLATES 



Trypanosoma, especially when the "cultural" forms of the last-named 

 are compared with those of the other two (or with what Novy and 

 M'Neal regard as their equivalents the Insectan forms). In the 

 case of the Trypanosomes, there is the same relative position of the 

 two nuclei, either close together, or the kinetonucleus even on the 

 flagellar side of the trophonucleus ; while certain of them show no 

 sign of an undulating membrane, but have a typically Herpetomonad 

 facies. Novy and M'Neal, in fact, would include all these types in 

 the genus Trypanosoma. 



Further, we may point out that according to the view which 

 these authors themselves hold regarding the origin of the blood- 

 Trypanosomes, it is most natural to suppose that they are to be 

 met with, quite at home, in an Insectan host. The American 

 workers say that parasitism in the living blood is to be looked upon 

 as the result of previous adaptation to the more or less digested 

 blood (in the Invertebrate). (As will be seen later, we agree with 

 this view, where certain Insects are the Invertebrate hosts.) Now, 

 in this course of evolution of certain blood-Trypanosomes, it may 

 be reasonably inferred that at one stage the parasites still remain 

 connected with the Invertebrate after having gained a footing in the 

 Vertebrate (say a bird). The question would seem to be, rather, 

 which if any blood-forms so descended have lost the ability to live 

 (and develop) in their Invertebrate host a course which would 

 probably greatly restrict their opportunities for dispersal. (In this 

 connection the case of the Leishman-Donovan bodies is most 

 instructive ; cf. pp. 258, 259.) 



Hence, summing up, there can be little doubt that certain of 

 these parasites of mosquitoes, especially those with trypaniform 

 characters, are connected with some Vertebrate host, just as are 

 those of other blood-sucking Invertebrates. At the same time, it is 

 also probable that some of the (typical) Herpetomonads found (e.g. 

 those occurring in larvae, such as Patton's form, also certain forms 

 described by the Sergents) are simply and primarily parasites of 

 the Insect. Lastly, it is, of course, possible that such a parasite 

 may have developed a trypaniform condition as an adaptation to 

 the food of a sanguivorous Insect> without, however, having become 

 able to live in the Vertebrate host ; but so far no example of such 

 a case is definitely known. And this brings us to the subject of 

 the derivation of the Trypanosomes. 



(b) Evolution and Phylogeny. 



It must be fully recognised that any views which can be at 

 present advanced upon this interesting, but very puzzling topic are 

 at best little more than speculations. Formerly (I.e.), the writer 

 inclined to the idea that all Haemoflagellates are to be derived from 



