CLASSIFICATION OF PATHOGENIC TRYPANOSOMES 521 



passages and round corners in which bacteria would become impacted. 

 It has been shown that filters which are impermeable to bacteria on 

 filtration will, nevertheless, allow bacteria to grow through them if 

 sufficient time for multiplication is allowed. It is quite possible that in 

 the experiments of Reich and Beckwith the positive results depend upon 

 the altered trypanosomes in the tissues of dead animals being more 

 plastic and even smaller than those in living animals. Such forms are 

 probably in a degenerate condition, though capable of revival if brought 

 into a favourable environment such as occurs when they are inoculated 

 into a living animal. 



Classification of the Pathogenic Trypanosomes. — Many attempts have been 

 made to separate the pathogenic trypanosomes from one another 

 on purely morphological grounds. To a certain extent this can be 

 done, but there are many named species which cannot be recognized 

 from their morphological features alone. Thus, there exist many trypano- 

 somes which are structurally like T. evansi, but have been separated by 

 cross-immunity tests and differences in the susceptibility of laboratory 

 animals. It seems to the writer that it has not yet been proved that these 

 tests are of specific value. It would be a remarkable circumstance if 

 T. evansi, which in its natural home in India produces surra in horses, 

 had not spread to other countries in view of the extent to which horses 

 have been moved from one part of the world to another during the last 

 two or three hundred years. Many of the trypanosomes of North Africa 

 and South America, for instance, resemble T. evansi in their morphology, 

 but have been separated from it as distinct species by the methods men- 

 tioned above. It seems more probable that these are merely races of 

 T. evansi which have been slightly modified by local conditions after long 

 separation from the parent stock. The same remark applies to the various 

 trypanosomes of the T. congolense group. If this view is adopted, then the 

 pathogenic trypanosomes can be grouped in species according to their 

 morphological characters, and the very similar forms which reveal dif- 

 ferences in regard to immunity and virulence for laboratory animals only 

 may be considered as races of these. On this basis it is possible to recognize 

 in the tsetse-fly areas of Africa the following forms: T. brucei (including 

 T. rhodesiense) in man and animals, T. gafubiense in man and occasionally 

 in domestic animals, T. congolense and T. simice in animals, T. vivax in 

 animals (once found in man), T. cajproe and T. uniforme in animals. In 

 tsetse-free areas of Africa, as also in other parts of the world, there is 

 T. evansi or its races and T. equiperdmn in animals. 



The pathogenic trypanosomes are often spoken of as being either 

 polymorphic or monomorphic. In this connection the terms apply purely 

 to the appearance of the trypanosomes in the vertebrate host or in inocu- 



