and that when he gets it the attack is usually mild; and also that the yellow-skinned races are more susceptible than the negro, but less so than the European. It was also said that the susceptibility of the European increases in proportion to the height of the latitude of his native place; that is, the Norwegian is more susceptible than the Frenchman, and the Frenchman more than the Italian or the Spaniard. The facts on which this belief was founded may admit of another and more rational explanation than that of varying degrees of racial susceptibility; relative racial susceptibility may be more a matter of racial opportunity than of colour of skin. We now know that the negro from a non-endemic place—Barbados, for example— is just as liable as the European to an attack of yellow fever should he visit an endemic or epidemic area.
Incubation period.— The incubation period of yellow fever rarely exceeds four or five days; it may, it is said, be much shorter— under twenty-four hours. The limits, according to Bérenger-Féraud, are one to fifteen days in the temperate zones, one to thirty days in the tropics. Precise experiments indicate an average incubation of from three to five days, and an extreme limit of thirteen days. Occasionally it happens that the disease breaks out in a ship after she has been several weeks at sea, having had no communication with the land or with another ship in the meantime. The rationale of this, and also of the prolonged incubation periods formerly assigned by writers to the disease, we now understand. It must not be inferred from these ship epidemics that the incubation period is to be reckoned in weeks. Infected mosquitoes may have been in the ship from the time she left port, but the crew may not have been bitten and infected until long afterwards.
Incubation period preceding epidemic extension.— It has been observed that a period of at least a fortnight elapses between the arrival of a yellow-fever patient in a hitherto uninfected district and the occurrence of the first case of the epidemic he may give rise to. That is to say, that although, as stated
