156 NATURAL HISTORY ESSAYS 



to be swarming with microscopic parasites of the 

 genus Trypanosoma, and after death the heart and 

 lungs are markedly anaemic. Wild animals 

 Burchell zebra, sable antelope, and so forth are 

 not liable to this terrible malady : an infusion of 

 zebra blood, however, is not sufficient to save the 

 victim, judging from the loss of some zebra-horse 

 hybrids which were experimentally inoculated in 

 1898, and died eight weeks afterwards. 1 Strange 

 to say, although adult oxen readily succumb to the 

 disease, the sucking calf is immune from the 

 poison. 



In consideration of the enormous pecuniary loss 

 sustained every year from these two plagues, it 

 is evident that an inestimable benefit would be 

 conferred on civilisation if some animal could be 

 found capable of acting as an efficient substitute 

 for horse or ox, and of course immune from either 

 disease. The common donkey is only partially 

 liable to the tse-tse poison, and has been 

 frequently employed : but on account of its 

 inferior size and self-willed disposition, this animal 

 cannot be considered as an effective substitute 

 for either horse or ox. A very interesting series 

 of experiments was made in the Transvaal during 

 1893, when eight wild-caught Burchell zebras 

 (Eguus burchellii) were trained to run in mail 



i See Proceedings of the Fourth International Congress of Zoology : 

 Cambridge, 1898. 



