The restriction as regards altitude, however, probably depends upon the fact that the mosquito which serves as an intermediate host is a coast species, which does not live in elevated regions.  It is a well-established fact that yellow fever has never prevailed in the City of Mexico, although the city has constant and unrestricted intercourse with the infected seaport, Vera Cruz.  Persons who have been exposed in Vera Cruz during the epidemic season frequently fall sick after their arrival in the City of Mexico, but they do not communicate the disease to those in attendance upon them or to others in the vicinity.  Evidently some factor essential for the propagation of the disease is absent, although we have the sick man, his clothing and bedding, and the insanitary local conditions which have been supposed to constitute an essential factor.  I am not aware that any observations have been made with reference to the presence or absence of Culex fasciatus in high altitudes, but the inference that it is not to be found in such localities as the City of Mexico seems justified by the established facts already referred to.

As pointed out by Hirsch, “the disease stops short at many points in the West Indies where the climate is still in the highest degree tropical.”  In the Antilles it has rarely appeared at a height of more than seven hundred feet.  In the United States the most elevated locality in which the disease has prevailed as an epidemic is Chattanooga, Tennessee, which is seven hundred and forty-five feet above sea level.

It will be remembered that the malarial fevers are contracted as a result of inoculation by mosquitoes of the genus Anopheles, and that the malarial parasite has been demonstrated not only in the blood of those suffering from malarial infection, but also in the stomach and salivary glands of the mosquito.  If the yellow fever parasite resembled that of the malarial fevers, it would no doubt have been discovered long ago.  But, as a matter of fact, this parasite, which we now know is present in the blood of those sick with the disease, has thus far eluded all researches.  Possibly it is ultramicroscopic.  However this may be, it is not the only infectious disease germ which remains to be discovered.  There is no doubt a living germ in vaccine lymph and in the virus from smallpox pustules, but it has not been demonstrated by the microscope.  The same is true of foot and mouth disease and of infectious pleuropneumonia of cattle, although we know that a living element of some kind is present in the infectious material by which these diseases are propagated.  In Texas fever, of cattle, which is transmitted by infected ticks, the parasite is very minute, but by proper staining methods and a good microscope it may be detected in the interior of the red blood corpuscles.  Drs. Reed and Carroll are at present engaged in a search for the yellow fever germ in the blood and in the bodies of infected mosquitoes.  What success may attend their efforts remains to be seen, but at all events the fundamental facts have been demonstrated that this germ is present in the blood and that the disease is transmitted by a certain species of mosquito—­Culex fasciatus.