176 Epidemics. 



through the medium of respiration. The two last writers 

 are some centuries later than either Aretseus or Galen, and 

 wrote at a time when the Mosaic disease was passing from 

 the infectious into one nearer to the modern form, which is 

 truly endemic and hereditary, but rarely infectious. 



This brief outline of the metamorphosis of the oldest 

 disease in the world will give some idea of the constant 

 changes which well-known diseases are prone to assume 

 after the lapse of ages. 



Modern pathologists, and may be nosologists, will main- 

 tain the invariability of disease ; as some zoologists maintain 

 the permanency of species in animals, not even admitting 

 an improved breed by crossing, or the admission might be 

 clothed in the language of a forced deviation from Nature 

 by selection, contrary to the ordinary plan of Nature, but 

 when left to itself would resolve itself back into the primary 

 form of either one or the other of its progenitors. 



But this is exactly the point aimed at, not to show that 

 cholera ever becomes yellow fever, or that typhus fever is 

 ever ague, though typhus fever may be nothing else than the 

 epidemic and infectious form of endemic typhoid fever. 

 The leading object designed is to show that vital power has 

 very singular selective powers, shown alike in ordinary 

 ailments, endemic and epidemic diseases ; but that, with this 

 great tendency to isolation in its destructive, or its preserva- 

 tive powers, yet that all diseases, in process of time, undergo 

 modifications in type, duration, and activity, from constrained 

 circumstances acting within, or upon the earth's surface ; 

 moreover, that changes pass over the whole globe, and 

 affect every kind and form of disease, and may-be the entire 

 of our vegetable economy in an almost imperceptible 

 manner, ranging over extensive eras, and probably not far 

 distant in duration from every 600 years ; but, according to 

 the nearest approximation, about every 640 years. 



