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* DISTRIBUTION OF ANIMALS. 313 DISTRIBUTION OF DISEASES. anil blemi, lorming transitional zones or debat- able grounds. The streteh of mountainous des- erts from the western Sahara to Manchuria is sueh a zone, where representatives of northern and southern faunas mingle, and where dwell many speeies not known elsewhere in either. The so-called "Sonoran' subreyion (southwest United States to the highlands of Central America) is another borderland of debatable validity. The distinction between the -Malayan and Australian subregions insisted ui>on by Wallace disappears entirely in respect to frogs and toads. In truth, the lesser subdivisions that have been so nu- merousl}^ and e.aetly niarked off by specialists have no real existence. "They depend," to use Gadow's words (1!102), "uijon the class, or even order, of animals which we may happen to study. The faiuiistic distribution of the Lrodela is not that of the Anura. and both follow separate lines of disjiersal, diflerent from those of the various ordsrs of reptiles, birds, and nianinials. This must be so. There is no doubt that the distribu- tion of land and water was totally diflerent in the Coal Age from what it is now. The face of the globe at the .lurassic Age can scarcely be com- pared with the aspect which the world had as- simied in the Miocene Period. This leads to an- other consideration often neglected. We know- that the various classes, orders, families, etc., of animals have appeared successively on the stage. A group which arose in the Coal Age followed lines of dispersal dift'erent from one which was not evolved until .Jurassic times, the post-Cre- taceous creatures could not avail themselves of what assisted their ancestors, and vice versa. Speaking generally, the older a group the more likely it is to be widely distributed. If it appears scattered, this may be due to extinc- tion in intermediate countries, or to submergence of former land connexions. . . . It is the raorphologist who is ultimately responsible for the establishment of faunistic regions, not the systematist, least of all he who accepts an elab- orate classification, and then mechanically, mathe- matically, by lists of genera and speeies, maps out the world." Bibliography. L. K. Schniarda, Die rieoijra- phische Verbreiturtg der Thiere (Vienna, 1S33) ; P. L. Sclater, Proceedings Linnwan Society (Zo- ology), vol. ii. (London, 18.57); id., The /his (London, 1891); A. Murray, Geographical Dis- iribulion of Animals (London, 1806) ; Thos. Hux- ley, Proceedings Zoological Society of London, for 1808; A. R. Wallace. The (leogriiphieal Distribu- tion of Animals (Xew York, 1870) ; A. Heilprin, The Geographi-cal and Geological Distribution of Animals (Xew York, 1887) : H. Gadow. chap- ter on "Geographical Distribution." in Bronn's Thierreich (Berlin, 180,3) ; A. Xewton, Diction- ary of Birds (London, 1893-96) ; F. E. Beddard, A Text-book of Zoogeography (Cambridge. 189.5) ; R. Lvdekker, Geographical Histoni of Mammals (Cambridge. 1890); W. L. and P. L. Sclater, The Geography of Mammals (London, 1800) : H. Gadow, Amphibia and Rr/, tiles (London. 1002). For Xorth America, con-ult especially Heilprin and Xewton, and the following: S. F. Baird, "Distribution and iligration of Xorth American Birds," in American Journal Sciences and Arts, series 2, vol. xli. (Xew Haven. 1866) ; .T. A. Allen, "Bird Faunse of Eastern Xorth America," in Bulletin Museum of Comparative Zoiilogi/, vol. ii. (Cambridge, Mass., 1871); Allen, "Geo- graphical Distribution of the Manunalia," in Bul- letin United States Geological Surrey (vol. iv., Xo. 2, Washington, 1878); Merriani, Proceed- ings Biological Society of W'ashinf/ton, vol. vii. (Washington, 1802) ; and -Biological Survey of the San Francisco Mountain Uegion," in orlh American Fauna, Xo. 3 (Department of Agricul- ture, Washington, 1800). DISTRIBUTION OF DISEASES. It is generally known that local conditions, such as climate, variatiuu of temperature, the composi- tion of the soil, elevation of tlie land above the sea-level, distribution of water, and llie character of the vegetation, combine with the peculiar habits of the people to determine the greater or less prevalence of certain diseases in the dill'erent regions of the earth. The science of nosography in this aspect did not receive much attention be- fore the beginning' of the nineteenth century, and is still comparatively undeveloped. The inipor tance of climate as a factor in the distribution of diseases is obvious. Yet certain diseases, as Eg)-ptiau ophthalmia, the pellagra of Lombardy, the beri-beri of Ceylon and the Malabar coast, and the elephantiasis of the Indian peninsula, ap- pear to have no relation to climate, but are con- fined to certain well-defined districts. Tropical regions are the home of malarial fevers, cholera, and hepatic diseases. This is due in part to the damp soil and decaying vegetation, particularly in the river valleys. The yellow fever of the ilexican Gulf, though often aggravated by other conditions, doubtless originated primarily from this cause. In the more temperate zones, typhus, typhoid, intermittent, and scarlet fevers are found. They are, however, for the most part, not so much endemic as epidemic. In the Xorth- ern Hemisphere, north of the tropical zone, ca- tarrhal diseases prevail, while in the correspond- ing zone of the Southern Hemisphere they are unknown. Intestinal catarrh prevails, however, to a considerable extent in some parts of the intertropical regions. In some cases, hilly regions are ravaged by fevers, while in the inter- vening valleys fevers seldom occur. The cultiva- tion of the soil sometimes essentially modifies the character of the malarial diseases. ' The destruc- tion of forests often results in the introduction of diseases unknown before. Indeed, as a gen- eral rule, living vegetation tends to preserve health, while decaying vegetation is a prolific source of disease. Defective drainage, natural or artificial, is also a common source of disease, especially in cities. The personal habits of races and communities in respect to diet and cleanli- ness, exert a wide influence upon the public health. Europe, on the whole, possesses the requisite conditions of health in greater perfec- tion than any other quarter of the world. The rates of mortality from diseases of the lungs are greater in northern than in southern latitudes. This is illustrated by (he prevalence of consunip- tion in the northeastern portion of the I'nited States. Fevers are more ])revalent in the South- ern than in the Xorthern States. Malarial fevers are especially fatal in the southern regions of the country; they are infrequent, however, where pine forests abound. Consult; Annesley, Researches into the Causes of Diseases in India (London, 18,55) ; Martin, Influence of Tropical Climates on European Con- stitutions (London, 1877) ; Manson, Tropical