LOGICAL BASIS OF MOSQUITO-REDUCTION 91 



The life of gnats, like that of other animals, is governed by fixed 

 laws. Propagation 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. Diseases, predatory animals, unfavorable condi- 

 tions, and accidents depress the density of population; and in fact 

 local reduction, that is, artificial depression of the density of popu- 

 lation, practically resolves itself into (a) direct destruction and 

 (6) artificial creation of unfavorable conditions. 



Let us now endeavor to obtain a perfectly clear 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 

 equally favorable to the propagation of gnats (or of any other 

 animal) ; and suppose that every point of it is equally attractive to 

 them as regards food-supply ; and that there is nothing such for 

 instance as steady winds or local enemies which tends to drive 

 them into certain parts of the country. Then the density of the gnat 

 population will be uniform all over the country. Of course, such a 

 state of things does not actually exist in nature ; but we shall never- 

 theless find it useful to consider it as if it does exist, and shall after- 

 wards easily determine the variations from this ideal condition due 

 to definite causes. Let us next select a circumscribed area within this 

 country, and suppose that operations against the insects are under- 

 taken inside it, but not outside it. The question before us is the 

 following: How far will these operations affect the mosquito-density 

 within the area arid immediately around it? 



Now the operations may belong to two categories those aimed 

 at killing the insects within the area, and those aimed at checking 

 their propagation. The first can never be completely successful; 

 it is in fact impossible to kill every adult winged gnat within any 

 area. But it is generally possible to destroy at least a large pro- 

 portion of their larvae, which, it is scarcely necessary to remind you, 

 must live for at least a week in suitable waters, and which may 

 easily be killed by larvacides, or by emptying out the waters, or 

 by other means. This method of checking propagation consists, 

 in the case of these insects, of draining away, filling up, poisoning, 

 or emptying out the waters in which they breed. Obviously the 

 ultimate effect is the same if we drain away a breeding-pool or if 

 we persistently destroy the larvae found in it; though in the first 

 case the work is more or less permanent, and in the second demands 

 constant repetition. If we drain a breeding-area we tend to pro- 

 duce the same effect at the end of a year as if we had destroyed 

 as many gnats as otherwise that area would have produced dur- 

 ing that period. Thus, though we cannot kill all mosquitoes within 

 an area, even during a short period, we can always arrest their 



