SMALLPOX AMONG SHEEP IN WILTSHIRE. 733 
speculation, where ingenuity must supply the place of more 
sober research. 
When, moreover, a contagious element is manifestly pre¬ 
sent, we err egregiously if to this we attribute the sole part 
in the propagation of disease. No word, in the whole range 
of medical science—human or veterinary—has done, or still 
does, more mischief than the word £i contagion.” Given 
contagion , and it is too commonly supposed, both profession¬ 
ally and popularly, that in this single fact we have all that is 
required to account for the propagation of a communicable 
disease. It cannot be too strongly insisted upon that conta¬ 
gion is not an absolute but a relative fact; that the contagious 
element is inoperative except under certain conditions. 
Remove the conditions, and the poison-germ is powerless. 
No specific communicable affection is exempted from this 
law, not even smallpox or scarlet fever, although these 
diseases are least manifestly subjected to it. It is not suffi¬ 
cient, therefore, to determine, in investigating the extension 
of a communicable disease, that it possesses contagious or 
infectious properties, but also to determine under what con¬ 
ditions those properties come into play. Now, it is in re¬ 
searches conducted to this latter end that all the great dis¬ 
coveries and improvements in the control and prevention of 
epidemics have taken place, and which still promise the 
richest results to the investigator. Until a very recent period 
it has been customary to lay the greatest stress upon, and to 
devote the most attention to, the exclusion of the poison-germ. 
Even now, largely in medical and more especially in veterinary 
science, the chief aim is concentrated upon measures to 
prevent the introduction of the contagious element. There 
can be no doubt that if this could be effected, the greatest 
conceivable protection from a disease would be obtained. But 
the practicability and efficiency of such a course must be 
learned from experience. What, then, does experience teach 
us on this subject? It teaches us that the strictest quaran¬ 
tine and the most perfect sanitary cordon have not prevented 
either the introduction, or spread, or development, of any one 
of the great epidemic or contagious diseases in a state or 
country ; that whenever, over large districts or in isolated 
localities, the attention has been centred solely upon the pre¬ 
vention of the introduction of contagious disease from without, 
that prevention has, as a rule, failed; that the most virulently 
contagious diseases have broken out within the limits of 
and in spite of the most stringent protective observation, 
while contiguous districts and populations, entirely unpro¬ 
tected, have either escaped or suffered but very slightly. On 
the other hand, experience has also taught us that the 
