394 
Mammalian Trypanosomes of A frica 
associated with G. palpalis, in the dietary of which fly he still plays an im¬ 
portant part in many districts. To-day the polymorphic trypanosomes of 
palpalis areas are of comparatively low virulence in the primitive food 
mammals of such regions—man and his stock and such game as may be 
present. Reptiles are, as far as is known, immune under natural conditions; 
and the rarity of trypanosomes in the large number of hippopotami examined 
by various observers suggests that this animal is in a marked degree tolerant, 
if not practically immune. These palpalis-trypanosome infections run a more 
or less chronic course in laboratory animals, and the parasites themselves 
show no posterior-nuclear forms in so far as they have been examined in this 
respect. Game-tsetse or morsitans areas are characterised, on the other hand, 
by polymorphic trypanosomes of the brucei type, to which, it appears, man 
has been immune until recent years—the tolerant hosts being essentially 
the game. Stock may be reckoned as relatively susceptible, since natives in 
many parts of Africa have from time immemorial avoided grazing their cattle 
in the proximity of big game. The morsitans trypanosomes cause an acute 
infection in laboratory animals, and tend to show posterior-nuclear forms. 
With one exception— T. pecaudi of Roubaud—all the polymorphic organisms 
whose development in Glossinae has been studied, have their “anterior 
station” in the salivary glands of the fly. 
(28) Let us for a moment suppose that, instead of a number of species 
of polymorphic trypanosomes differing from one another according to bio¬ 
metrical, serological, or laboratory pathogenicity tests, there existed originally, 
throughout the tsetse areas of primitive Af rica, a single species, characterised by 
polymorphism, and by its development in the salivary glands of Glossinae the 
mammalian hosts of this organism would differ, we may suppose, in different 
regions—according to the food proclivities of the prevailing tsetse species. 
In palpalis areas, under primitive African conditions, the fly would feed 
mainly on man and his domestic animals, and on game, where it was available. 
In morsitans and pallidipes areas, under similar undisturbed conditions game 
would be selected. In each type of fly-area, natural selection would tend to 
establish tolerance to the trypanosomes on the part of those mammals which 
constituted the main food supply of the particular tsetse concerned with their 
transmission in that area. Introduction of new mammals into the food 
regimen of the tsetse might react in three different ways on the trypanosome: 
(a) the parasite might be unable to survive in the new host, in which case 
it would die out; (b) it might give rise to an acute and rapidly fatal disease, 
in which case it would automatically become extinct; or (c) it might find the 
new host suited to its requirements— i.e. tolerant—in which case it would 
survive and become established. 
(29) Hitherto we have considered chiefly the effects produced upon the 
trypanosome by sojourn in different vertebrate hosts. But in parasites such 
as these, with a life-history normally passed in two different hosts—verte¬ 
brate and invertebrate—we must also take into account the possible effects 
