THE ADVANCE OF SCIENCE 141 



Another feature of the progress of our knowledge of 

 disease as a scientific problem is the recent recognition 

 that minute animal parasites of that low degree of uni- 

 cellular structure to which the name ' Protozoa ' is given, 

 are the causes of serious and ravaging diseases, and 

 that the minute algoid plants, the bacteria, are not alone 

 in possession of this field of activity. It was Laveran 

 a French medical man who, just about twenty-five 

 years ago, discovered the minute animal organism in 

 the red blood-corpuscles, which is the cause of malaria 

 (see fig. 44). Year by year ever since our knowledge of 

 this terrible little parasite has increased. We now know 

 many similar to, but not identical with it, living in the 

 blood of birds, reptiles, and frogs (see fig. 45). 



It is the great merit of Major Ross, formerly of the 

 Indian Army Medical Staff, to have discovered, by most 

 patient and persevering experiment, that the malaria 

 parasite passes a part of its life in the spot-winged gnat 

 or mosquito (Anopheles), not, as he had at first supposed, 

 in the common gnat or mosquito (Culex), and that if 

 we can get rid of spot-winged mosquitoes or avoid their 

 attentions, or even only prevent them from sucking the 

 blood of malarial patients, we can lessen, or even abolish, 

 malaria. 



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 



