690 



SCIENCE. 



[N. S. Vol. XXII. No. 570. 



remind you that the gnat acts as an inter- 

 mediary, becoming infected when biting 

 infected persons and, some weeks later, in- 

 fecting healthy persons in its turn — the 

 parasite passing alternately from insect to 

 man. The hypothesis 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 re- 

 ducing gnats in given localities and con- 

 sider that the point has been proved by 

 experiment, others are much more sceptical 

 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, pos- 

 sibly a continued loss of life by the diseases 

 occasioned by them ; and I, therefore, pro- 

 pose 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 per- 

 sonal impressions as to their numbers be- 

 ing small or large; but I am aware of no 

 criterion by which we can express those 

 numbers in actual figures. We can not 

 anywhere state the exact number of mos- 

 quitoes to the square mile or yard, and we 

 can not, therefore, accurately gauge any 

 local decrease which may have resulted 

 from operations against them. A method 

 of doing this may be invented in the fu- 

 ture; 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. Un- 

 like particles of matter (so far as we know 

 them) the living unit can not 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 determination 

 by proper analysis ; and the logical position 

 of the question of local reduction depends 

 upon this analysis. 



The life of gnats, like that of other ani- 

 mals, is governed by fixed laws. Propaga- 

 tion can never exceed, nor mortality fall 

 below, certain rates. Local conditions may 

 be favorable either to the birth rate or to 

 the death rate; and the local population 

 must depend upon the food supply. Dis- 

 eases, predatory animals, unfavorable con- 

 ditions and accidents depress the density 

 of population ; and in fact local reduction, 

 that is, artificial depression of the density 

 of population, practically resolves itself 

 into {a) direct destruction and (6) arti- 

 ficial creation of unfavorable conditions. 



Let us now endeavor to obtain a per- 

 fectly cleaj picture of the problem before 

 us by imagining an ideal case. Suppose 

 that we have to deal with a country of 

 indefinite extent, every point of which is 



