CLIMATIC RANGE OF YELLOW FEVER. 67 



with the infection upon them, and the disease did not 

 spread to the residents of Jalapa. As Sir James Martin 

 has pointed out "It would seem that the diseases of 

 tropical climates, like certain vegetable productions, 

 are restricted to certain altitudes and particular degrees 

 of temperature." * 



There can be no doubt that this is so, though the 

 confidence of the public in this assertion is from time 

 to time shaken by the occurrence of these cases, brought 

 up to the hills by persons in whose systems the 

 disease is already latent. But to give, if possible, still 

 greater emphasis to Sir James Martin's maxim, we may 

 here mention the very remarkable fact that the range 

 of altitude to which yellow fever ascends in Mexico 

 and other places, is found to be coincident with that 

 of certain plants. In the present instance it has been 

 ascertained that as a rule, " this disease ceases at that 

 altitude at which the Mexican oak ( Quercus Xalapensis) 

 commences to appear, that is at 2750 feet." f 



Now, it is well known that temperature is the con- 

 dition which, more than anything else, governs the 

 range of plant life and though the matter is by no 

 means as yet very well understood, many of these 

 diseases of malarial origin are supposed to be dependent 

 upon germs of a parasitic origin, and, according to 

 competent authority, yellow fever " is dependent for 

 its origin and spread, upon a temperature not lower 

 than 70 Fahr." 



At lower temperatures therefore, it is probable that 



* The Influence of Tropical Climate in Producing Disease, by Sir 

 James Martin, 2nd edit, 1861, p. 283. 



f Stanford's Compendium of Geography and Travel for Central 

 and South America. Edited by H. W. Bates, 1878, p. 73. 



Quain's Dictionary of Medicine, p. 1798; Article "Yellow Fever." 



