134 THE AMERICAN MONTHLY [May 
There are many diseases of which it may be said that 
in our time and in our country they arise only by conta- 
gion. Prominent among these are smallpox, scarlet fever, 
measles, cholera, tuberculosis, glanders, bovine pleuro- 
pneumonia, foot-and-mouth disease, and rabies. Record- 
ed history does not tell us where and under what circum- 
stances the first case of any of these diseases appeared, 
any more than it tells us where and under what circum- 
stances the first dog appeared.’ We know by observation, 
and by observation alone, how dogs are propagated at the 
present day, and we accept observation as conclusive up- 
on this point. Why should we not accept observation 
and experimentation as conclusive in regard to the pro- 
pagation of a contagious disease ? 
While we can not resonably expect at this late day to 
decide the cause of contagious diseases by speculation as 
to the first appearance among animals of such diseases, it 
is legitimate to make such an inquiry in order to obtain 
a better understanding of these plagues. Science has 
made great progress in explaining the origin of species, 
and even in tracing in general terms the development of 
life upon earth ; and while it can not say definitely where, 
when, and how the dog originated, it has been made plain 
that in some prehistoric age the dog developed from some 
earlier and related animal form, not by a sudden trans- 
formation, but by gradual transition. And in the same 
manner this early ancestor of the dog developed from a 
still earlier ancestor, doubtless quite different from the 
dog as he is to-day. To be brief, in tracing the develop- 
ment of the dog, we should be obliged to go back, step 
by step, toward the dawn of creation, toward simpler and 
simpler forms of life, until the primordial germ is reach- 
ed. Just where in this long series of succeeding forms or 
just when in the countless ages that have elapsed since 
the beginning of the series the disease known as rabies 
appeared it it impossible to say. It may have been in 
