Considerable variations are noticeable in the appearance of 
mdividual trypanosomes of any strain : we have the so-called ‘ male ’ 
and 'female ’ elements; and gross measurements are found to vary. 
But,be it noted, these variations are within limits. It would be most 
exceptional to find, for instance, any T. et’ansi of less than 20/1 or of 
more than 35/1 in length ; and despite the tens of generations which 
man y trypanosoiiic strains have produced in various laboratories, we 
find the morphological features remain practically constant. There is 
not that tendency to develop new types, to revert to an old form, or 
otherwise to vary, which has more than once been suggested. We 
may with comparative safety, then, group into one class some of 
those nine species which conform approximately to the dimensions of 
T. rt-ansL Disregarding the dilTcrenccs in ‘ male, ‘ female and 
'indifferent' forms, these species would be mononior\)hic and of 
relatively large size. 
On the other hand, there is a group of three species the morpho¬ 
logical appearances of which differ markedly. In the course of 
infection by T. diworphon, forms closely resembling T. eianst are 
encountered, but at the same time, or in the same animal, or, capable 
of production in another animal, trypanosomes markedly smaller, 
measuring only lOfi to 15^ in length, and without any free flagellum, 
make their appearance. It has been shown that these are but various 
manifestations of the same organism, which is consequently far rom 
being monomorphic. 
Of the nine species to which we refer, six show similar react 
towards experimental laboratory animals; three differ marvC > 
The value of animal reactions may be disputed, and when limitc 
amount, results are apt to cause confusion. I aiticularly is t ■ 
rase with the donkey, cattle, sheep and goats, largely, we t im 
owing to the multitude of races and breeds employed, and also because 
niany of them are country-bred animals whose ancestors 
probably been exposed for generations to trypanosome 1 
These animals, too, show a degree of individual idiosyncrasy 
prone to give rise to error. Monkeys, especially Cercofithec ■ 
rAwttx, domestic rabbits, guinea-pigs, white rats and / 
to a slightly less degree, dogs of both European and ntrtrve blood, are 
however, fairly constant in their susceptibility or I eir insu 
to any one strain of trypanosome. During the many generation 
