THE FIGHT AGAINST YELLOW B'EVER. 347 



spreading over a very large part of the warmer portion of the inhab- 

 ited globe. Thus, if an infected vessel, having on board men or 

 merely mosquitoes contaminated with the disease, makes a landing in 

 (he 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 scoj^e and in duration ; the port, the city, the comitry is 

 ravaged. This is what happened in Spain at the time of the epi- 

 demic 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, Avhich exhausts itself on the spot. The in- 

 fectious 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 tlie case 

 with the yellow-fever invasion observed at JMarseille, at St. Xazaire, 

 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 jDarallel, 

 and consequently the disease does not flourish there. 



III. 



This line of demarcation between the countries that are susceptible 

 (>f infection and those which are not, a line fixed by the forty-third 

 parallel, has much importance in the campaign against the yellow 

 fever. On one side of the boundary the j^eril 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 simpli- 

 fied, (hi 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 theorv and 

 practice. How the obscurities disappear I How the paradoxes van- 

 ish that troubled investigators just a few years ago ! The physicians 

 in the INIarseille (juarantine during the epidemic of 18*21 understood 

 nothing of the nature of that disease which was so frightfully con- 

 tagious 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 IDOO " have taught us that there 



o See Smithsonian Report for 1901. ]ip. r)r)7-<!T."5 : also bio<:rai)hy of Dr. Walter 

 Iteed in present Smithsonian Report for V.)iC>. 



