This great discovery was followed by another as to the production of the deadly ‘Nagana’ horse and cattle disease in South Africa by a screw-like, minute animal parasite _Trypanosoma Brucei_ (see fig. 46 B). The Tsetze fly (see fig. 48 A, B), which was already known in some way to produce this disease, was found by Colonel David Bruce to do so by conveying by its bite the Trypanosoma from wild big-game animals, to the domesticated horses and cattle of the colonists. The discovery of the parasite and its relation to the fly and the disease was as beautiful a piece of scientific investigation as biologists have ever seen. A curious and very important fact was discovered by Bruce--namely, that the native big game (zebras, antelopes, and probably buffaloes), are _tolerant_ of the parasite. The _Trypanosoma_ grows and multiplies in their blood, but does not kill them or even injure them. It is only the unaccustomed introduced animals from Europe which are poisoned by the chemical excreta of the Trypanosomes and die in consequence. Hence the wild creatures--brought into a condition of tolerance by natural selection and the dying out of those susceptible to the poison--form a sort of ‘reservoir’ of deadly Trypanosomes for the Tsetze flies to carry into the blood of new-comers. The same phenomenon of ‘reservoir-hosts’ (as I have elsewhere called them) has since been observed in the case of malaria; the children of the native blacks in Africa and in other malarious regions are _tolerant_ of the malarial parasite, as many as 80 per cent. of children under ten being found to be infected, and yet not suffering from the poison. This is not the same thing as the immunity which consists in _repulsion_ or _destruction_ of the parasite. [Illustration: FIG. 45. _Lankesterella ranarum_ (Lank.), the parasite of the red blood-corpuscles of the edible Frog, described originally as _Drepanidium ranarum_ by Lankester in 1882, and previously without name in 1871. The large ovals represent the red corpuscles of the frog; the dark central mass is the nucleus, N. In A two spindle-shaped parasites are seen; in B one larger parasite with nucleus _n_ preparing to divide; in C the parasite is =V=-shaped. In D the parasite has become spherical, and is so in E also. In F the spherical parasite has divided into a number of spores _mz_, with a central residual body _np_. The figures G to N represent supposed stages in conjugation of small and large forms; O is an encysted phase; and P a spore or sporozoite of the sexual generation similar to the needle-shaped exotospores of Laverania. (See Fig. 44.) All the figures magnified 2,250 diameters. (After Hintze from Minchin’s section on Sporozoa in Lankester’s “Treatise on Zoology.”)] [Illustration: FIG. 46.