732 
SMALLPOX AMONG SHEEP IN WILTSHIRE. 
and then only as a guide to research, not a limitation. At 
every sten we are taught caution in the use of analogical 
argument in pathology. The purulent ophthalmia of infants 
may unquestionably be occasioned in parturition, and there 
is no doubt that, once developed in a single individual, it may 
be widely propagated to others by the direct application of 
the purulent discharge, and probably also by diffusion of 
particles of this discharge through the atmosphere. It can 
scarcely be admitted that the causes of the first and subse¬ 
quent cases are identical. The same is true of acquired and 
propagated purulent ophthalmia in man and of certain forms 
of gonorrheal discharge. 
It is undoubtedly correct that we do not know the limit of 
retention of deleterious properties by contagious or infectious 
matters. In smallpox and scarlet fever, for example, particu¬ 
larly the latter, we know that the poisonous emanations from 
the sick maintain their active properties for long periods. 
But we must not confound the question of limitation with 
that of indestructibility. It is to be remembered that the 
contagious virus diffused from the sick has never yet been 
separated from an infected atmosphere, and subjected to 
scientific analysis. Much may be inferred from its effects; 
but much also lies hid. If we permit ourselves, then, to 
adventure conclusions other than those which are warranted 
by the facts actually within our grasp, we run the risk of end¬ 
less confusion. 
The occasionally observed prolonged retention of virulent 
properties by the poisonous emanations or secretions of cer¬ 
tain specific diseases must, in seeking the conditions deter¬ 
mining any given outbreak of such diseases, not be lost sight 
of, and must, among other considerations, influence our 
inquiries. For example, the poisonous emanations or se¬ 
cretions from epizootic pleuro-pneumonia or eczema will long 
infect stables, cattle-sheds, trucks, and even plots of ground 
But it is manifest that to extend the conclusions from this 
fact beyond what is actually warranted by observation can 
aid us little. For where we must, in the absence of all other 
evidence, fall back upon the doctrine of the “ indestructi¬ 
bility” of specific poison-germs for the explanation of any 
given outbreak, it is tolerably certain that the immediate 
conditions of the outbreak are so much more important than 
the theoretical that we may well afford to let the latter rest 
in abeyance. The doctrine of “ spontaneity’ 5 may never, 
perhaps, extend beyond this point, that, under certain con¬ 
ditions, all evidence of specific origin to a disease is lacking; 
but if the doctrine be made to imply more than this, like 
that of “ indestructibility/ 5 it carries us into a region of 
