ment of its vital functions. The perfect insect can subsist only at a temperature between 60 and 100° F.; below 60° it is paralyzed, benumbed, it dies; at 65° it moves with difficulty. It bites with energy only above 75°. It mates between 68 and 85°, but there is fertilization only when the temperature is above 75°. It lays its eggs in the water where it lives, in stagnant water of flower vases, gutters, bottles, tubs, and sinks, but only when the temperature is from 80 to 85°.

The conditions of development of the larvæ are not less rigorously precise. The development of the egg and the hatching of the larva demand a temperature between 68 and 85°-the best is at 82°. All lowering under that figure shows itself by a greater or less retardation in their development. The larva is aquatic. Its respiration of air obliges it to fix itself at the surface of the water or to rise there to seek the air periodically. It develops normally into a perfect insect in a period of nine days, provided the temperature at night does not go below 80°; otherwise the formation of the winged insect, capable of mating and of reproducing itself, is prolonged to forty or even sixty days.

To sum up, then, it appears that the yellow-fever mosquito obtains its full and regular vital development only at an average temperature of 82°, and that any lowering of the temperature night or day renders less efficient some one of its physiological functions. This data is consequently of the first importance. The strict dependence of the mosquito on temperature conditions is an essential fact in the interpretation of the history of yellow fever; it is the key to all its mysteries. The Stegomyia needs plenty of heat and of heat steadily maintained. As soon as the temperature falls, the insect's life becomes endangered, and at 60° it is benumbed and soon dies. This sensitive insect does everything in its power to protect itself from the increasing chilliness of the air, and its mode of living offers the means for doing so. It is practically a fellow-boarder with man, under the same roof. It is a domestic animal, like the house fly. Whenever it is cold, it takes refuge in kitchens, bathrooms, heated bedrooms, in bakeries, or in other warm places. On ships it finds a last resort close to the engine room, near the heat pipes or smoke pipes. If the temperature falls below 60°, it becomes torpid and benumbed like a marmot.

These conditions in their rigorous precision are, so far as known, peculiar to the Stegomyia among all mosquitoes, and explains many features in the history of yellow fever. I shall mention but one. This relates to the peculiar immunity enjoyed by the inhabitants of Petropolis, in Brazil. Petropolis may be called the sanitarium of Rio Janeiro. It is a country resort 30 miles

SM 1905-26

from the capital, at an altitude of 2,800 feet. It is the residence of the well-to-do population-the diplomatic corps, the principal merchants, financiers, and government officials. Before nightfall all these take the tram for the elevated region, where they find bracing air and healthful security. Petropolis, in fact, is free from yellow fever at the very time when Rio is scourged with it, and yet between these two places there is a lively traffic and constant relations. Even more, some yellow-fever patients are taken there for treatment throughout their illness, yet the disease is not communicated to anyone. The reason is that the Stegomyia can not live in that climate, and such as come in the railway trains each day perish immediately, for the evenings are cool and during the night the temperature often falls below 60°. There is sometimes another explanation given for the immunity from fever enjoyed in this Brazilian country resort. It is attributed to the altitude of the region above sea level. This is a mistake. Yellow fever and its mosquito are found even at greater heights than Petropolis. It is enough to mention the epidemics at Morne Rouge, in Martinique (920 feet elevation); at Camp Jacob, in Guadeloupe (1,800 feet); at Newcastle, in Jamaica (4,000 feet). The fact is that thermometric conditions are paramount.

It is found that the extreme heat and humidity best adapted to the welfare of the yellow-fever mosquito are most common along the coasts of tropical countries. The lands best suited to the swarming of this insect and to the spread of the plague form a belt around the earth north and south of the equator. If lines be drawn in the northern and southern hemispheres corresponding to the forty-third parallel of latitude, the circles would mark the upper and lower limits of the home of the Stegomyia and, accordingly, of the yellow fever. The region comprised in this vast zone form what MM. Chantamesse and Borel call the "infectible territories," and the more temperate countries on either side of this zone, denied to the mosquito, are the uninfectible territories." The Stegomyia fasciata considered as a species can not become acclimated beyond this equatorial zone, for it does not find that almost invariable temperature of 82° which is indispensable for the proper exercise of its vital functions, and especially of reproduction.

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The infectible zone represents the actual or virtual habitat of the infectious mosquito. It is the region of epidemics and is also the region of possible endemic or permanent centers of infection. At present the Stegomyia, which is a genus of mosquito very cosmopolitan in its habit, lives in widespread districts throughout the infectible zone. Theobald found it in India, the Malay archipelago, in Japan, in Africa, in America, and in all warm countries where he hunted for it. The contagion is capable, if not well guarded, of

spreading over a very large part of the warmer portion of the inhabited globe. Thus, if an infected vessel, having on board men or merely mosquitoes contaminated with the disease, makes a landing in the infectible zone, it threatens a whole country with an epidemic; men inoculate the Stegomyia, and new generations of Stegomyia carry the disease to other men; the plague progresses, the contagion spreads in scope and in duration; the port, the city, the country is ravaged. This is what happened in Spain at the time of the epidemic at Cadiz and at Barcelona in 1804 and 1812.

The case is totally different in the uninfectible territories situated beyond the habitat of this mosquito. The infected vessel occasions only a local epidemic, which exhausts itself on the spot. The infectious insects, on account of their sedentary habits, never wander far from the ship that houses them; they bite only those imprudent enough to disturb them. At the farthest they move only to the neighboring vessels. Since the climate is unfavorable for their reproduction, their ravages last only during their ephemeral life. Hence these minor epidemics are limited to a single ship or to an anchorage basin and vanish of their own accord. This was the case with the yellow-fever invasion observed at Marseille, at St. Nazaire, at Swansea, and in general at all French and English ports. The reason for this is understood. It is because all of England and nearly all of continental France are beyond the forty-third parallel, and consequently the disease does not flourish there.

III.

This line of demarcation between the countries that are susceptible of infection and those which are not, à line fixed by the forty-third parallel, has much importance in the campaign against the yellow fever. On one side of the boundary the peril is great, and sanitary measures should be rigorous. On the other side there is, so to speak, no danger at all, and the subject of sanitation is very much simplified. On either side of this entomological and pathological frontier the health regulations may, and should, differ. They should be made in accordance with scientific facts, which enlighten both theory and practice. How the obscurities disappear! How the paradoxes vanish that troubled investigators just a few years ago! The physicians in the Marseille quarantine during the epidemic of 1821 understood nothing of the nature of that disease which was so frightfully contagious on shipboard and which ceased to be so the moment patients were transferred to the city hospital. All is clear now that scholars on the United States commission of 1900 have taught us that there

a

See Smithsonian Report for 1901, pp. 657-673; also biography of Dr. Walter Reed in present Smithsonian Report for 1905.

is no contagion from the infected persons, nor from their clothing, nor even from their fæces, but only by infected mosquitoes, which, in the case mentioned, were confined on a vessel. What a paradox it seemed to epidemiologists of that time that of two cities-Barcelona and Marseille-one clean and new, the other insanitary and old, it was the first which was visited by the yellow fever, while the second remained untouched. There is no longer a paradox for those who know that one city is on the northern and the other on the southern side of the frontier that limits the habitat of the contaminating mosquito.

The forty-third parallel strikes the continent of Europe at Ferrol, in Spain, follows the Pyrennes districts, crosses the Hyères Islands below Marseille to the heights of Leghorn in Italy; it leaves below it nearly all of Spain, the southern half of Italy, and of the French possessions part of the Hyères Islands and of Corsica. Care should be taken in directing to any one of these islands any French colonial companies returning from yellow-fever countries.

The infectible countries, possible prey for the pestilence, comprise, as may be seen, a considerable enough portion of Europe, the whole of Africa, a large part of Asia, of Australia, and the oceanic islands. Here is an immense empire that must be watched. It will become more and more formidable, as direct relations are multiplied with the different endemic centers of Brazil, the Antilles, and the Gulf of Guinea. The cutting of the Isthmus of Panama, by opening to the plague Polynesia and the Asiatic world, might create a menacing danger.

The way to prevent this extension is to attack the agent of its propagation, the Stegomyia, both on land and sea; on land by trying to purge the endemic centers where the insect becomes infectious, and on sea by attempting to destroy the mosquito itself in the ships where it finds refuge.

Three signal victories have been gained over yellow fever during these later years-in Cuba, in Brazil, and in Dakar, in West Africa. The first is the most memorable of these events. It is the purification of the endemic center at Habana. This occurred in 1901, during the United States occupation. The daily press in countless articles has spread the details. We know that Brig. Gen. Leonard Wood, governor of Habana, decreed one fine day that the plague should be wiped out and the mosquitoes destroyed throughout the entire city of Habana and its suburbs, and we know that it was done. Praise has rightly been given to the spirit of decision, of activity, of energy, and even Draconian rigor which attended the execution of this work. It remains to point out its wisdom, its exact conformity to scientific theories.

The theory was that the mosquito is the sole disseminator of the disease. This is precisely what the United States commission, appointed the year before, had just proven. It had shown that all the other supposed causes of contagion were imaginary; that a man could sleep in the bed of a sick patient or of one deceased, could come in contact with his fæces, put on his clothes, use his linen, confine himself in badly ventilated rooms at a humid temperature of 100°, and leave unscathed by the test if he escaped the mosquito bite. The extermination of the plague, then, leads to the extermination of the mosquito. But this pretension of banishing such a wily enemy seems foolish at first sight. You can hardly rid a room of one little insect that buzzes around, and yet they say you would rid a swampy country of legions of mosquitoes that abound there.

The yellow fever Stegomyia does not breed in swamps. It has not the habits of the Anophele of the marsh, the malaria mosquito. It does not live like that one, in the open country, but dwells in houses. It is a domestic insect. It stays at home, is wary, and is sensitive to the weather. Like many other mosquitoes, it never goes more than 500 or 600 yards away from its breeding place and journeys only when its home—a vessel or a carriage-journeys. There is no need to fear that the insect may be carried far by the wind, for it dreads the wind. It does not trust itself outdoors when there is the lightest breeze. The problem is thus simplified. It is no longer a question of protecting immense areas. It is enough to protect the house and its immediate environs-the city and a limited surrounding zone. Still it would be useless to capture the insect on the wing or at rest. It is permitted to complete its short life, but is not allowed to have offspring. The female is prevented from laying its eggs. This is accomplished by draining stagnant water left in so many gardens and household utensils where the mosquito seeks a breeding place. Hence the efficacy of the measures which forbade the people of Habana from keeping water in any other way than in covered receptacles or with a coat of oil or petroleum on top.

The success of the measures taken by the American physicians, Gorgas, Finlay, and Guiteras, in Habana was complete. Yellow fever has disappeared from there. On April 4, 1904, the President of the Republic of Cuba, in his message to the Congress, spoke thus:

There has not been in Cuba since 1901 a single case of yellow fever not imported. The country should know of this excellent sanitary condition, which is due to the perfection of prophylactic measures and the vigilance of the health authorities.

Events happened in the same way in Brazil. Dr. Oswaldo Cruz, in charge of the organization of the campaign against yellow fever, with equal success repeated at Rio de Janeiro what had been done in