Page:Popular Science Monthly Volume 23.djvu/664

646 as a probable cause of malarial fever, in place of the marsh-vapors of Lanscisci ("New Orleans Medical and Surgical Journal," vol. iv, pp. 563-601, 1848). And, even before his time, I find that a paper on the "Mosquital Origin of Malarial Disease" was published by Dr. John Crawford in a periodical known as the "Baltimore Observer," 1807, no copy of which I have yet been able to get hold of.

I now propose to present a series of facts—some of the best known and most generally established facts—with regard to the so-called "malarial poison," and to show how they may be explicable by the supposition that the mosquito is the real source of disease, rather than the inhalation or cutaneous absorption of a marsh-vapor. These facts are, briefly, as follows:

1. "Malaria affects, by preference, low and moist localities"—in fact, swamps, fens, jungles, marshes, etc. This statement no one will dispute. Conformably with it we find the mosquito does the same. The female lays her eggs, to the number of two hundred and fifty or three hundred, in a boat-shaped mass, on the surface of any natural or artificial receptacle for fresh water. Early in spring the larvæ are found in the bottoms of pools and ditches, feeding upon decaying matter (hence the works on entomology state that they are of great benefit in clearing swamps of miasms(?)). These larvæ are the so-called "wrigglers," or "wigglers," to be found in great numbers in any stagnant pools of water during summer. They change into pupæ, and, in a few days more, the pupa-skin is cast, and floating on this latter, like a raft, the insect finally takes flight, a full-developed gnat. Many thousands perish by drowning, or are devoured by fish while extricating themselves from their pupa-cases. As the eggs develop into perfect insects in three or four weeks, many broods are hatched during the warm season, which accounts for their increasing numbers during the later summer and autumnal months. Some species deposit their eggs in soft mud or in dry sand, but all require moisture in the larval state.

2. "Malaria is hardly ever developed at a lower temperature than 60° Fahr." A temperature of 60° F. is necessary for the development of the mosquito.

3. "The evolution or active agency of malaria is checked by a temperature of 32° F." The mosquito is killed or paralyzed, so that its active agency is checked, by a temperature of 32°.

4. Malaria "is most abundant and most virulent as we approach the equator and the sea-coast." The swarms of mosquitoes (as well as of sand-flies, ants, and other insect-plagues) that infest many equatorial regions are well known; and, with regard to sea-coasts, the accumulation of mosquitoes is both a fact and easily susceptible of explanation. Under the influence of gentle land-breezes the mosquito is wafted toward the ocean, but, in the absence of strong winds that