640 IDENTITY OF PLEURO-PNEUMONIA AND RUBEOLA. 
istic, that they never occur in isolated cases, nor ever fall 
upon a whole community suddenly and simultaneously. They 
are always epidemic. But epidemics are of two kinds—the 
non-infecting and the infecting. The former happens when, 
as is supposed from some unknown change in the air, or in 
the magnetism of the earth, a morbific influence suddenly 
seizes a large proportion of the inhabitants, laying them for 
a time prostrate in sickness, and then with the cessation of 
general exciting cause as suddenly and simultaneously disap¬ 
pearing. A good example of this form of epidemic is seen in 
any ordinary invasion of influenza. It is not infectious, and 
one attack gives no security against any number of subse¬ 
quent attacks, when exposed to the same causes. The 
second form of epidemic is that in which an equally unknown 
morbid element is brought amongst a people by some person 
under its influence, from whom it is transferred to others, and 
from them propagated over all who are susceptible, until the 
whole are infected. This form of epidemic rages irrespective 
of season or of atmospheric or telluric conditions, until all 
suffer who come within its influence, when it dies out, for 
want of pabulum, as it were. Examples are too well-known 
in the histories of epidemics of measles, scarlatina, smallpox, 
&c. Cases do sometimes appear apparently sporadic, just as 
the epidemics themselves are often apparently capricious; but 
these instances are so exceptional as to leave doubt of authen¬ 
ticity, and, under any circumstances, do not invalidate the 
rule. It is unnecessary to add that affections of this form of 
epidemic rarely occur more than once in the same individual; 
or that one, if not two, of them may be induced artificially in 
a modified form, giving equal security against the spon¬ 
taneous occurrence of the severe form or type of the natural 
disease. If the experience and knowledge, now ample, 
acquired relative to these facts by long study of them, as they 
occur in the human body, be transferred to observation of 
and for comparison with the various forms of disease occur¬ 
ring among the lower animals, they will be found equally ap¬ 
plicable in many cases, and inferentially in all. As animals 
have a physiology in common with the human race, so have 
they a pathology. The unity of design in structure and func¬ 
tion is equally manifested in health and in disease. What is 
true of epidemics is ,mutatis mutandis,equ?\\y true of epizootics; 
what of endemics, of enzootics. The diseases are not only 
closely allied, they are identical — (i Mutato nomine detefabula 
naratur All the fevers incidental to humanity are repro¬ 
duced, not only in the equine and bovine, but in all other 
species brought under the same physical conditions. Thedis- 
