MOSQUITOS AND MALARIA. 219 



Europe which present these supposed conditions, but in a sparsely- 

 populated country like Africa they may be found on every hand. 

 In Mashonaland we have numberless areas of this description, 

 but perhaps the most typical are to be found in the Umfuli Dis- 

 trict, where there are considerable tracts of land rendered unin- 

 habitable owing to the fact that they contain no surface-water 

 during the dry season, although during the rains they may be 

 sometimes under water for several weeks at a time ; and yet 

 these very localities are among the most unhealthy in the 

 country. Again, in the Northern Transvaal there are large 

 stretches of totally unpopulated country, more especially where 

 the brackish rivers prevail ; and yet it is a matter of common 

 knowledge that the Boer hunters who used to penetrate into 

 these wildernesses from the healthy high veldt invariably suffered 

 severely from malarial fever. 



Another conclusion which must be drawn from the theory as 

 it now stands is that in any malarial region where Anopheles is 

 generally distributed the most unhealthy portions must neces- 

 sarily be those in which the population is thickest. This con- 

 clusion is practically the exact reverse of what is actually the 

 case in this country, at least so far as the white population 

 is concerned. For example, Salisbury is by far the most densely 

 populated portion of Mashonaland, and, further, I have found 

 three species, if not four, of Anopheles in the town, one of which 

 is certainly common ; moreover, not only is a mild form of fever 

 prevalent in the district, but also all the severer cases of fever 

 from the lower veldt are brought into the town for treatment, 

 thus creating a continual supply of foci for the distribution of the 

 disease. And yet despite all this there can be absolutely no 

 question that Salisbury is far more healthy than the vast majority 

 of the sparsely populated districts which surround it, although in 

 these latter the chances that a mosquito will carry malarial con- 

 tagion from one man to another are necessarily more remote, 

 and, indeed, often infinitesimal. 



In fine, the theory proposed by Grassi amounts to this : 

 given Anopheles and a certain number of malarial patients, then 

 locality goes for nothing in the causation of malaria ; a position 

 which is, to my mind, untenable. 



The foregoing objections do not in any way militate against 

 the supposition that Anopheles is the sole carrier of malaria, but 

 only affect the proposed explanation as to how the mosquito itself 

 is infedted. Other objections, however, are raised on the ground 

 that there is reason to believe that fever can be acquired other- 

 wise than through the agency of mosquitos. 



The argument most generally adduced is that ploughing is in 

 itself a very unhealthy occupation in malarial districts, more 

 particularly where new ground is being broken up, and that in 

 such cases mosquitos cannot reasonably be regarded as a factor. 



