5 
many eminent men that several diseases now regarded as Non-zymotic 
will have to he added to the list.* 
Before entering on the immediate subject of the paper I will mention 
the following characteristics of the whole class, which to my mind are 
fully proved. 
h They are never spontaneously developed hut propagated from person 
to person. All the conditions supposed to he favourable to them existed among 
the red Indians from unknown antiquity, yet these diseases never appeared 
amongst them until they were communicated to them by the white man. 
2. As a rule, they affect an individual but once in life. One attack 
protects the system against another attack of the same disease, but not 
uniformly ; this protection varies in different individual members of the 
class. A second attack of Small Pox is rare, whilst a second and a third 
attack of Typhoid Fever is not uncommon. 
3. One species of disease is no protection against another however 
allied to it in general character. I have known Typhoid, Typhus, and 
Small Pox, succeed one another at short intervals in the same person. 
When Typhoid Fever cases are admitted into a ward containing true 
Typhus, convalescents from the former frequently contract the latter, and 
so with the whole members of the class. 
4. "We know the conditions favourable to their development when the 
germs are present but we know nothing of their absolute origin, we can 
only trace their history and from their analogies deduce certain 
conclusions. 
At an early period of my professional life I was much struck and 
puzzled at the resemblance between different diseases showing, as it seemed 
to me, a strong family relationship. A further acquaintance with the 
history of diseases proved that I was not alone in this respect, that it had 
taken mankind centuries to separate and classify certain members of the class, 
and that two of the most important, viz. — Typhus and Typhoid — had 
come down to the present decade of the 19th Century without being 
clearly recognized as distinct species. Accurate observers in England, 
France, and Scotland, have described the differences for a long period, but 
it did not strike them that these indicated a difference of species. I have 
classified some of these diseases according to their supposed affinity. I am 
not at all satisfied that it is the best classification, but let it suffice if it but 
serve to illustrate the subject and call forth further enquiry. 
* Pulmonary Phthisis, Cancer, &c— See Dr. William Budd's paper on 
the subject, in the " Lancet." 
