90 PREVENTIVE MEDICINE 



gnat acts as an intermediary, becoming infected when biting infected 

 persons and, some weeks later, infecting healthy persons in its turn 

 the parasite passing alternately from insect to man. The hypo- 

 thesis that the infection in these diseases may be produced in any 

 other manner than by the bite of gnats has not been justified by 

 any recorded experiments or by any substantial arguments ; and we 

 may, therefore, assume for the present that if we could exterminate 

 the intermediary agents, the gnats, in a locality, we could also 

 exterminate there the diseases referred to. But here we enter upon 

 ground which in the opinion of many is much less secure. While 

 some believe in the possibility of reducing gnats in given localities 

 and consider that the point has been proved by experiment, others 

 are much more skeptical and hold that the experiments were not 

 sound. This state of uncertainty naturally causes much hesitation 

 in the adoption of measures against gnats, and, therefore, possibly 

 a continued loss of life by the diseases occasioned by them; and I, 

 therefore, propose to sift the matter as carefully as time will allow. 



In the first place, we should note that experiments made in this 

 connection have not been very satisfactory, owing to the fact that 

 no accurate method has yet been found for estimating the number 

 of gnats in any locality. We can express our personal impressions 

 as to their numbers being small or large; but I am aware of no 

 criterion by which we can express those numbers in actual figures. 

 We cannot anywhere state the exact number of mosquitoes to the 

 square mile or yard, and we cannot, therefore, accurately gauge 

 any local decrease which may have resulted from operations against 

 them. A method of doing this may be invented in the future; but 

 for the present we must employ another means for resolving the 

 problem one which has given such great results in physics 

 namely, strict logical deduction from ascertained premises. 



As another preliminary we should note that mosquito-reduction 

 is only part of a larger subject, namely, that of the local reduction 

 of any living organisms. Unlike particles of matter (so far as we 

 know them) the living unit cannot progress through space and time 

 for more than a limited distance. The diffusion of living units 

 must, therefore, be circumscribed a number of them liberated 

 at a given point will never be able to pass beyond a certain distance 

 from that point ; and the laws governing this diffusion must be the 

 same for all organisms. The motile animal is capable of propelling 

 itself for a time in any direction; but even the immotile plant calls 

 in the agency of the winds and waters for the dissemination of its 

 seeds. The extent of this migration, whether of the motile or the 

 immotile organism, must to a large degree be capable of determina- 

 tion by proper analysis; and the logical position of the question of 

 local reduction depends upon this analysis. 



