594 
POPULAR SCIENCE REVIEW. 
of self-propagation can ever be generated de novo. It is 
maintained^ on tbe one band^ tbat such an occurrence is as 
impossible as the spontaneous generation of plants or ani- 
mals ; while^ on the other hand, it is argued that the poison 
of certain diseases capable of self-propagation may, under 
certain favourable conditions, be produced independently of 
any pre-existing cases of the disease. The comparison of 
a fever-poison with a spore or ovum is an ingenious, but a 
most delusive, argument. An epidemic disease springing up 
in a locality where it was before unknown, and where it 
is impossible to trace its introduction from without, is said 
to be not more extraordinary than the development of fungi 
in a putrid fluid. The argument, however, is founded on a 
pure assumption, for there is not a tittle of evidence to show 
that a fever-poison is of the nature of a spore or ovum. Air 
saturated with the poisons of various contagious diseases has 
been condensed and submitted to the highest powers of the 
microscope, but nothing approaching to a small-pox spore, or 
a typhus . ovum, has yet been discovered. It is true that 
certain contagious diseases, such as scarlet fever and small- 
pox, can in most instances be traced to contagion ; but, with 
regard to others, such as typhoid or enteric fever, it is in 
most instances utterly impossible to account for the first cases 
in any outbreak on the theory of contagion, while, at the same 
time, there is direct evidence that the contagious power of the 
disease is extremely low. The question is no doubt beset 
with many difficulties, and constitutes one of the most intri- 
cate problems in medical science. It is one, however, which 
can never be solved by entering on the discussion with a pre- 
conceived theory as to the close analogy, if not identity, of a 
fever-poison with an animal or vegetable ovum, nor by assum- 
ing that the laws which regulate the propagation of one con- 
tagious disease are equally applicable to all. Nature^s facts 
are too often interpreted by human laws, rather than by the 
Taws of nature. In the case before us, the natural history of 
each disease must be studied independently, and our ideas as 
to its origin and mode of propagation must be founded on the 
evidence furnished by that study alone, and irrespective of 
the laws which seem to regulate the origin and propagation of 
other diseases with which it has no connection whatever, except 
in the human mind. 
At the present moment, when the subject of epidemics is 
attracting so much attention, it may be interesting to call 
attention to the more important diseases comprised under that 
head, and to point out some of the main facts connected with 
their origin and distribution. The principal epidemic diseases 
then are, small-pox, scarlet fever, measles, typhus, relapsing 
