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SCIENCE. 



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



take shelter, the circles of Diagram I. 

 would become ellipses with the generating 

 pool as a focus. In such a case the wind, 

 and especially devious winds, would have a 

 distributive tendency; but it must be re- 

 membered that if the insects are scattered 

 further apart their members at a given 

 point must be reduced. A wind which 

 blows mosquitoes into an area must blow 

 others out of it. The net result of devious 

 winds on a circular drained area would be 

 that the mosquito-density is not so much 

 reduced at the center, but is reduced to a 

 greater distance outside the boundary circle 

 — so that the average reduction remains the 

 same. With a wind blowing continuously 

 from one direction, the indication would 

 be to extend the drainage further in 'that 

 direction. Obviously, wind may scatter 

 mosquitoes ; but it can not create them, nor 

 prevent the total average reduction due to 

 anti-propagation measures, as some people 

 seem to think. It is, however, very doubt- 

 ful whether wind does not really drive or 

 scatter mosquitoes to any great degree. In 

 my experience. they are extremely tenacious 

 of locality. Thus Anopheles were seldom 

 seen on Tower Hill, a low open hill in the 

 middle of Freetown, Sierra Leone, al- 

 though numerous generating pools existed 

 a few hundred yards from the top, all 

 around the foot of it, and the winds were 

 often very strong. If a continuous wind 

 can drive mosquitoes before it, then during 

 the southwest monsoon in India they should 

 be driven away from the west coast and 

 massed towards the east coast; but I have 

 never heard that they are at all less numer- 

 ous on the west coast. I have often seen 

 very numerous mosquitoes on bare coasts 

 exposed to strong sea-breezes, as at Madras. 

 As a rule, they seem to take shelter in the 

 presence of a strong breeze. Instances of 

 their being driven far by winds are fre- 

 quently quoted, but in my opinion they 



were more probably bred, in many such 

 cases, in unobserved pools close at hand. 

 The wind-hypothesis is frequently used by 

 municipal officials as an excuse for doing 

 nothing — it is convenient to blame a marsh 

 miles distant for propagating the mosqui- 

 toes which are really produced by faulty 

 sanitation in the town itself. 



Another and similar statement is often 

 made with all gravity to the effect that 

 mosquitoes are brought into towns in trains, 

 carts and cabs. So they are ; but a mo- 

 ment 's reflection will assure us that the 

 number introduced in this manner must 

 always be infinitesimal compared with 

 those that fly in or which are bred in the 

 town itself. Moreover, if vehicles may 

 bring them in they may also take them out. 



I will now endeavor to sum up the argu- 

 ments which I have laid before you — I fear 

 very cursorily and inadequately. First I 

 suggested that there must be for every liv- 

 ing unit a certain distance which that unit 

 may possibly cover if it continues to move 

 all its life, with such capacity for move- 

 ment as nature has given it, always in the 

 same direction. I called this distance the 

 limit of migration. It should, perhaps, be 

 called the ideal limit of migration, because 

 scarcely one in many billions of living units 

 is ever likely to reach it — not because the 

 units do not possess the capacity for cover- 

 ing the distance, but because the laws of 

 chance ordain that they shall scarcely ever 

 continue t(\ move always in the same direc- 

 tion. Next I endeavored to show that, 

 owing to the constant changes of direction 

 which must take place in all random migra- 

 tion, the large majority of units must tend 

 to remain in or near the neighborhood 

 where they were born. Thus, though they 

 may really possess the power to wander 

 much further away, right up to the ideal 

 limit, yet actually they always find them- 

 selves confined by the impalpable but no 



