SMALLPOX AMONG SHEEP IN WILTSHIRE. 
731 
“ Is that possible or feasible ? In the presence of all the known facts 
respecting contagious diseases, we must express our entire disbelief in the 
theory of spontaneity. The same law extends to contagious diseases as is 
afforded in the propagation of animal life, i. e. there can be no new pre¬ 
sence without an antecedent, or, in plainer words, something cannot arise out 
of nothing. The first of these sheep attacked, at some particular moment, 
was a healthy animal; it became diseased, and as a result of that disease 
yielded products poisonous in character, which produced the same changes 
and the same products in other sheep : ergo , the first sheep must either have 
been subjected to a similar poison, or the others did not suffer from the 
contagion of the first. To admit any difference in this argument is to 
assume that two causes can produce one phenomenon, which is absurd. 
The spontaneous origin of the disease is, therefore, the last hypothesis that 
can be accepted; before it there are two others, either of which, however 
improbable, have at least the merit of not being impossible. The first of 
these is to the effect that the germ of the disorder was conveyed to the 
animal first affected, a germ derived from some other animal previously 
affected. How the germ was carried it would be speculative to suggest, 
but the possibilities of conveyance are within the range of experience. 
These poisons, indestructible at common temperatures, and solid in their 
form, have a susceptibility of adherence to other solid bodies, and may be 
intentionally or accidentally carried, retaining their poisonous properties for 
any number of years, over the whole universe; consequently, although the 
probabilities were as a million to one against such conveyance, the possibili¬ 
ties remain. 5 ’ 
The writer then proceeds ingeniously to suggest the pos¬ 
sible relationship of the disease which had broken out among 
Mr. Parry’s sheep to farcy. We presume, however, that now 
any doubt as to the actual nature of the affection, and as to its 
being variola ovina , is fully set at rest. 
A logical deduction holds good only so far as the premises 
upon which it is based are sound. Now, the assumption that 
the propagation of animal life and of contagious diseases is 
governed by the same law, is open to serious question. The 
analogy is, no doubt, most plausible; but this is not sufficient 
to justify the assumption. Recent experiments have again 
conclusively shown (a doubt having once more arisen on the 
subject) that animal life is never known to be generated 
without a presumption, amounting almost to absolute demon¬ 
stration, of a pre-existing germ. But we are not aware that 
any like proof, or equivalent probability, has been shown of 
the pre-existence of germs of at least two diseases, one of 
which, under certain conditions, is highly infectious, and the 
other is both contagious and infectious in the utmost degree— 
we allude to typhus and puerperal fever. These instances 
alone serve to vitiate the analogy and its logical conse¬ 
quences. Probability, not possibility, is the rule of analo¬ 
gical argument in physical science; and the possibility of 
the laws of one series of phenomena being similar to the laws 
of another and very different series (so far as at present 
known) cannot be admitted without the extremest caution, 
