INSECTS AND DISEASE. 649 



swamps and pools drained, so that the niosquito can not so readily find 

 a place suitable to deposit her eggs. And, as the forests and under- 

 brush disappear before the implements of the agriculturist, colonies of 

 mosquitoes, wafted from a distance by winds, are not " obstructed " 

 and " accumulated " by foliage, nor can the insect so readily escape, as 

 before, the numerous fly-catching birds that feed upon it. Even here, 

 however, artificial pools, tanks, and excavations containing water, may 

 constitute mosquito-nurseries from which many millions may be de- 

 veloped in a single summer. 



11. Malaria usually keeps near the surface of the earth ; it is said 

 to " hug the ground " or " love the ground." When blown by the 

 wind, however, or drifting up ravines, it has been known to rise sev- 

 eral thousand feet. Dr. Russell, in his address before the New York 

 Public Health Association, April 13, 187G, stated that, " under ordi- 

 nary circumstances, a certain altitude affords immunity from malaria, 

 although low elevations of 200 or 300 feet above a miasmatic tract are 

 often more dangerous than the flat lands, the poison seeming to float 

 upward and become intensified." This, he says, has long been no- 

 ticeable on the heights of Bergen Hill, "West Hoboken, and Wee- 

 hawken, which overlook the Jersey flats. In accordance with the 

 malarial vapor theory, these facts are completely mysterious. The 

 mosquito, on the other hand, is known to hover near the ground (or 

 water) from which it springs, and, being wafted by winds, can readily 

 be understood to be " obstructed " and " accumulated " by forests on 

 the brows of hills, etc. 



12. Malaria is most dangerous when the sun is down, and it seems 

 almost inert during the day. Of this there is no doubt, and the va- 

 rious hypotheses on the marsh-vapor theory, that have been alleged in 

 explanation of it, are almost as numerous as they are unsatisfactory. 

 With regard to the mosquito, however, it is well known that it re- 

 mains, for the most part, during the day, harbored in woods, weeds, or 

 low underbrush, and comes out after sunset and at night to indulge 

 its blood-sucking proclivities. 



13. The danger of exposure after sunset is greatly increased by 

 the person exposed sleeping in the night air. Again have the hy- 

 potheses based on the marsh-vapor theory been altogether insuscepti- 

 ble of explaining this circumstance satisfactorily. With regard to 

 mosquital inoculation, however, it is undoubtedly true that, while 

 awake, the person exposed will move about, or brush away the insect, 

 while he will submit to be bitten during sleep. 



14. In malarial districts, the use of fire, both in-doors, and to those 

 who sleep out, affords a comparative security against malarial disease. 

 Explanations on the marsh-vapor theory are numerous, various, and un- 

 satisfactory. With regard to the mosquito, however, it is well known 

 to be attracted by lamps, lights, and fires, into which it heedlessly flies 

 at the cost of life. In countries where these insects are extremely nu- 



