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736 at the present day is commonly connected with an unusual influx of strangers. It is not to be supposed, however, that the residents acquire immunity by acclimatization : they have no greater im munity in that sense than have the Bengalis from chole-ra, or than those who permanently reside in unsanitary localities from typhoid fever or diphtheria. The only immunity is that of the Negro race, an immunity all the more striking that the Negroes in yellow- fever ports are mostly found living in the favourite haunts of the disease, and that their race is peculiarly liable to all other infec tions of the kind, including cholera, typhus, and typhoid. Al though the protection of the Negro of pure blood is not absolute, it is nearly so. In ordinary circumstances Negroes become liable when of mixed blood, and almost exactly in proportion to the degree of white stock in their breeding. These racial -peculiarities have been so often remarked and are so universally admitted in their broad significance that no detailed proofs need be adduced. It may be stated, however, that all African Negroes, whether fresh from Africa or long acclimatized to the New World, have the same natural protection ; thus the Nubian regiment in the French ser vice during the Mexican expedition did not lose a single man, and did not even have a single case, in the epidemic at Vera Cruz in 1866. The protection is profoundly racial, and not due to the Negroes being inured to yellow-fever localities. There is no other instance of the same racial immunity in the whole range of in fective sickness. Theory Two things stand out prominently in the foregoing recital of of the facts, (1) that yellow fever, in time and place, has dogged the disease, steps of the African slave trade, and (2) that the African Negro has a very large racial immunity from yellow fever. The first of these facts was generalized by Audouard (Paris, 1825), but has been neglected and forgotten ; the second fact, which no one seriously disputes, is the complement and confirmation of the historical and geographical induction. The circumstances in Peru, although ap parently in contradiction, are really corroborative : a form of yellow fever established itself at certain ports of that country in the wake of a notorious Chinese coolie trade (see, ); but the Chinese themselves were exempt from the fever at the time, and would appear not to have suffered from it in the subsequent epidemics on the Peruvian coast. The question thus arises as to the particular connexion between the African slave trade (or the analogous Chinese coolie trade) and yellow fever. The first point is that the fever has not been a fever of the voyage but of the land ing place, although there are several authentic instances (e.g., cases of the &quot;Regalia&quot; and &quot;La Pique&quot;) of yellow fever arising at sea from the exposure of white men to the stench of a shipful of Negroes. Again, the filthy condition of a Guineaman on her arrival from Africa was a notorious fact ; and the filth of that kind was discharged into the creeks, carenages, and anchorages of slave ports in material quantity year after year for a long period. At Havana as many as a hundred slavers would arrive in one year. Steady accretions of the filth of slave-ships from the beginning of the traffic to America down to its abolition in 1808, and its final cessation previous to 1860, would account for a peculiarly pestiferous state of the harbour mud, of the beach, and even of the water ; in fact, the water in the Bay of Havana was pestiferous and full of organic matter where it was several fathoms deep, and there was a standing order in the British navy against admitting it into ships. Wher ever the harbours were most tideless, as around the Gulf of Mexico and in the West Indian Islands, and wherever the soil was most alluvial, and the movements of the ground-water most extensive, the specific putrefaction or fermentation thus introduced into the harbour would spread farthest on shore, being aided or encouraged always by the abundance of other organic matter which it met with at particular spots, such as the foundations of houses. The next step is to consider the connexion between this wholesale befouling of slave ports and the particular type of endemic disease. One of the points most emphasized by Audouard was the funda mental physiological differences between the African Negro race and the white. The discharges of the Negro body might become by their effluvia specifically poisonous, he argued, to white men under special circumstances. A more direct factor in the aetiology was the very common, if not uniform, prevalence of dysentery and diarrhoea in the passage from the Guinea coast to the western shores of the Atlantic : the filth in a slave-ship s bilges was, in part at least, dysenteric in its source and properties. Now, there is much independent evidence, collected from times of war and famine, to prove a certain correlation or equivalence between dysentery and typhus ; according to Blanc s phrase, the one was vicarious to the other. Yellow fever is admittedly a form of typhus, a form distin guished by a h;emorrhagic tendency. Thus we find the connexion explained between dysenteric and other evacuations of the Negro race, carried to the mud and alluvial soil of ports under very peculiar circumstances, and a special form of endemic typhus, dif fering from the ordinary form in being earth-borne instead of air borne. The remaining part of the synthesis concerns the differen tial type of yellow fever within the genus &quot; typhus : &quot; AVhence did it obtain the &quot; note &quot; or distinctive mark, anatomical and clinical, of an acute yellow atrophy of the liver ? The dysenteric filth that was imported into the harbours of the New World represented that almost unique unwholesomeness of life which is summed up in the phrase &quot;horrors of the middle passage.&quot; Among such horrors nostalgia, despair, and the sense of wrong were not the least ; and these are among the states of human feeling that have been known, now and then, in the ordinary way of life to assume that peculiar visceral embodiment, or to find that means of expression, which amounts to acute yellow atrophy of the liver, and stands, in fact, for a total arrest of the hepatic functions, biliary and other. There is no other theory of yellow fever to contest the field with the slave-trade hypothesis ; that alone satisfies all the conditions of a correct synthesis historical, geographical, ethnological, physio logical, and, some would say, even ethical. There have been various other theories put forward from time to time, all of them much too fragmentary, and therefore erroneous, such as the hypo thesis of rotting timber (in wharves, ships hulls, &c.), of sugar cargoes, of common foul bilge-water, of madrepores, and of bacilli. The bacillary or parasitic hypothesis is the fashionable one at present, but it is much too ambitious, as ordinarily held, and altogether wide of the mark. The part played by putrefactive organisms is a subordinate one. In the general grouping of factors they can only come in after we have found the specific integral of the yellow-fever soil in its endemic seats ; they cannot elaborate the miasmatic poison of yellow fever without a definite pabulum, any more than the &quot;lactic bacillus&quot; can produce lactic fermentation without milk-sugar. The sanitary or public health aspects of yellow fever have been Sanitr discussed in great part under. In tion. regard to its sanitation at the endemic seats in the West Indies, Guiana, Brazil, Central America, and the Gulf States of the American Union, the same principles apply as to other filth-diseases. The object is to secure a clean soil, and to that end drainage and sewer age serve best. In the sanitation of yellow fever the case is so far peculiar that the harbour bottom, the adjoining mudbanks and mangrove swamps, and even the seawater itself, are apt to retain the specific taint, especially where the cleansing action of the tides is slight. But there is good reason to think that the specific taint in the soil is everywhere slowly disappearing, now that it is no longer reinforced by fresh supplies year after year. It has practically vanished from the Atlantic ports of the United States, and has be come almost rare in such harbours as Port Royal, Jamaica. Its headquarters are now the Brazilian ports, which were the last to develop it (in 1849). Literature. The chief work is that of La Roche, Yellow Fever, 2 vols. , Philadelphia, 1855. A very full bibliography is given by Hiv.sch at the end of his section on &quot;Yellow Fever,&quot; in Historisch-Geographische Pathologie, vol. i., Stuttgart, 1881 (Engl. transl. by Creighton, London, 1883). Recent experience, epidemiological and clinical, is given in the writings (2 vols.) of Dr Joseph Jones of New Orleans, 18S7 ; see also Maclean s Diseases of Tropical Climates, London, 1886. Among the numerous monographs may be specially mentioned those of Pym (Observations -upon the Vulam Fever, London, 1815), who showed that one attack gave immunity, and of Daniel Blair (Some Account of the Last Yellow Fever Epidemic of British Guiana, with plates, 3d ed., London, 1852). A popular exposition of the slave-trade hypothesis of Audouard (with additions by the writer of the present article) will be found in North. Amer. Rev., October 1884. Audouard s three papers were collected under the title liecveil de Memoires sur le Typhus Nautique, ou Fievre Jaune, Paris, 1825. (C. C.)

 YELLOW RIVER. See, –.   YELLOWSTONE NATIONAL PARK, an area situated mainly in north-western,, which has been withdrawn from settlement by the United States Government and dedicated to the purposes of a public park. It is a region of hot springs and geysers, mountains and canons, lakes and waterfalls. While it is almost entirely comprised in, a narrow strip 2 wide projects on the north into , and on the west a strip about 5 in width projects into and into. Its boundaries, which were defined at a time when the country was little known, are as follows. The northern boundary is a parallel of latitude running through the mouth of Gardiner river, a branch of the Yellowstone, 2 north of 45° N. The eastern boundary is a meridian 10 east of the most easterly point of Yellowstone Lake, which places it almost on the 110th meridian. The southern boundary is a parallel 10 south of the most southerly portion of the same body of water, in 44° 10′N. The western boundary is a meridian 15 west of the most westerly portion of Madison (now Shoshone) Lake, this meridian being approximately that of 111° 6′. The Park is therefore very nearly a rectangle in shape, its length north and south being 61·8 and its breadth 53·6. Its area is 3312. Its surface is mainly an undulating plateau, with a mean