96 PREVENTIVE MEDICINE 



which governs their to-and-fro movements is that which governs 

 the drawing of blacK and red cards from a shuffled pack. The chances 

 against our drawing all the twenty-six black cards from such a pack 

 without a single red card amongst them are enormous, as are the 

 chances against a mosquito, guided only by chance, always wander- 

 ing on in one direction. On the other hand, just as we shall generally 

 draw black and red cards alternately from the pack, or nearly so, 

 so will the random movements of the living unit tend to be alter- 

 nately backward and forward tend, in fact, to keep it near the 

 spot whence it started. As there is no particular reason why it should 

 move in one direction more than another, it will generally end by 

 remaining near where it was. 



But it will now be objected that the movements of mosquitoes 

 are not guided only by chance, but by the search for food. To study 

 this point, take the diagram just given, place a number of pencil- 

 dots upon it at random, and suppose that each pencil-dot denotes 

 a place where the insects can obtain food suppose, for example, 

 that the breeding-pool lies in the centre of a large city and that the 

 pencil-dots are houses around it. Consideration will show that the 

 centripetal law must still hold good, because there is no reason why 

 the insects should attack one house more than another. There is 

 no reason why a mosquito which has flown straight from the pool 

 to the nearest house should next fly to another house in a straight 

 line away from the pool, rather than back again, or to the right or 

 left. The same law of chance will continue to exert the same influence, 

 and the insects will always tend to persecute most those houses which 

 lie in the immediate vicinity of their breeding-pool. Even when 

 there are many pools scattered about among the houses, there is 

 no reason why, after feeding, the mosquitoes will go to one rather 

 than to another; and the result must be that in general they will 

 tend to remain where they were. 



Self-evident as this argument may now appear, it is not under- 

 stood by many who write on the subject and who seem to think that 

 mosquitoes radiate from a centre and shoot forever onward into all 

 parts of the country as rays of light do. Accepting this fallacy with- 

 out question, they argue that it is useless to drain local breeding-pools 

 because of the influx of mosquitoes from without. Such an influx 

 certainly always exists; but I shall now endeavor to show that it 

 cannot generally compensate for local destruction. 



Let us consider a tract of country over which numbers of mos- 

 quito breeding-pools are scattered, with houses and other feeding- 

 places lying among them. Suppose \ve draw a straight line across 

 this country and drain away all the pools to the right of it, leaving 

 all those to the left of it intact. Then all the insects on the left of the 

 line must be natives of that part; and all those on the right of it 



