4 
II. 
On the Origin of Species in Zymotic Diseases. 
By David Davies, M.R.C.S. 
Medical Officer to the Local Board of Health. 
Read at the General Meeting, February, 1871. 
It is with some hesitation that I venture to bring this subject before this 
Society, knowing full well that in the present state of our knowledge the 
Usual means of elucidation, viz. : — the test tube, the microscope, and the 
pocket lens, are inapplicable to our subject; nevertheless, I consider 
Zymotic diseases a fit subject for the study of Naturalists, presenting as 
they do, most, if not all, the characteristics of the organic world, among 
which may be enumerated their power of self -propagation, their portability 
from place to place, their modification by seasons and years, the regularity 
of their manifestation, and the family relationship which exists between 
certain groups of them, indicating, as I take it, the development of certain 
divisions of them from some common prototype now possibly and most 
probably extinct ; and when we consider the part they have played in the 
history of our race, the havoc which they have made among the more 
recently discovered races of mankind,* the desolation and sorrow which 
they frequently carry into the bosom of our families, f I am quite sure the 
Members of the Naturalists' Society will excuse an attempt, however 
feeble, to throw some new light on the subject. 
The term Zymotic, derived from the Greek word Zume, a ferment, was 
adopted some years ago by the Eegistrar General, and is applied by him 
chiefly to seven diseases, which he considers to be the chief members of the 
class, viz. : — Small Pox, Scarlet Fever, Measles, Whooping Cough, Fever, 
including Typhus and Typhoid, Diphtheria, and epidemic Diarrhoea. 
These seven represent but a small number of the whole class; indeed, the 
outlines of the class are as yet not well denned, and it is the opinion of 
* See the late Dr. George Gregory's Lectures on "The Exanthemata." 
f " With regard to Zymotic diseases, it may be stated generally, that 
from 21 to 26 per cent, of the total number of deaths in Great Britain 
during a year are due to diseases of this class." — Ait ken's Science and 
Fractice of Medicine, page 210. 2nd Edition. 
Scarlet fever has, during the present decade, destroyed in three years 
over 80 thousand lives, — See different annual reports by the Eegistrar 
General. 
