F. 8. BILLLNGS. 
251 
Both forms are frequent; there is a constancy in the continuity of 
phe individuals attacked, and deport themselves as other infectious 
diseases of man and animals. The variola of man belongs to the 
true epidemics, that of sheep to the genuine epizootics. 
On the other side we have concluded that the remaining 
forms of variolas among the domestic animals do not form idio¬ 
pathic diseases, v. equina, vaccina (bovina), caprina, porcina, and 
canina, as they descend directly from one or the other of the 
above named protopathic forms, but may also at present take their 
origin reciprocally from one another. The most general charac¬ 
teristic for these' deuteropathic forms of variola is that they ap¬ 
pear unfrequently and never in epizootic form, generally more 
isolated or as enzootics limited to single stables or herds. 
We are at present unable to decide in what relation variola 
humana and variola ovina stand to each other, if one originally 
proceeded from the other, or which is the true original variola. 
Experimental pathology, which we have to thank for so many bril¬ 
liant results, has here an open and profitable field for further 
work. At least there is scarcely a doubt, and I here endorse in 
full the views of Bohn, that the reciprocal transmissability of the 
different forms of variola as well as the reciprocal substitution, 
vicarisation—“ Stettvertretung”—of the human and animal vari- 
olm indicate that at the foundation we have before us an identi¬ 
cal contagium, and that the variolse have sprung from a common 
matrix and are related to one another; as an individual inocu¬ 
lated with variolic contagium peculiar to another species is ren¬ 
dered non-susceptible not only against infection from its own 
peculiar form, but from that of other species, for a variable 
length of time. The probably originally identical variola conta¬ 
gium exerts a very different influence by the different zoological 
species, although in its principal effects it retains a certain degree 
of conformity. In this direction it presents a striking example, 
that contagii have to a certain degree the ability to conform 
themselves to the organisms in which they gain access—the man¬ 
ner is of no consequence—and thereby deport themselves conse¬ 
quently to the variations of the natural historical species, which 
pliable and capable of adaptation, a property which does 
