686 
“solution of the problem may, in the future, 
be built. The building of this foundation 
can not be recounted here; but it will be 
necessary to mention some of the materials 
of which it is made, that the latest progress 
may be intelligently discussed. 
Asin the case of smallpox, it had long 
been a matter of common observation that 
a number of the acute infectious diseases 
‘occur but once in the same individual. 
Whooping-cough, measles, scarlet-fever and 
yellow-fever are notable examples of acute 
infectious diseases one attack of which 
usually confers immunity against subse- 
quent attacks of the same disease. It was 
also observed that some infectious diseases 
confer a very evanescent type of immun- 
ity, and that others confer no immunity 
whatever. 
From the standpoint of immunity the in- 
fectious diseases may be easily divided 
into three classes : 
1. Diseases one attack of which confers 
immunity against subsequent attacks of the 
same disease. 
2. Diseases one attack of which confers 
immunity against subsequent attacks of the 
same disease for only short periods of time. 
3. Diseases an attack of which confers 
no immunity whatever. 
It would seem that these facts, coupled 
with Jenner’s discovery of a fundamental 
and practical method of producing artificial 
immunity, clearly outlined the path for 
future workers to follow; but, strange to 
say, the nineteenth century was well on its 
way before this important route found many 
followers. 
The failure to appreciate fully Jenner’s 
brilliant discovery, and to apply his method 
to the study of other infectious diseases, 
finds an explanation in the hazy theoretical 
conceptions of the cause and nature of in- 
fectious diseases which prevailed during the 
early part of the century. The investiga- 
tions of fermentation by Astier, Sette, Franz 
SCIENCE. 
[N. S. Vou. XIII. No. 331. 
Schulze, Cagnaird de Latour, Schwann, 
Fuchs, Remak, Mitscherlich, Helmholtz 
and others did much toward clearing the 
haziness of that period; but it was the 
monumental work of Pasteur that ‘ finally 
established the truth of the view that all 
processes of fermentation and putrefaction 
alike are caused by living things, and that 
in each different fermentation different 
kinds of microbes are concerned.’ In the 
light of newer knowledge this statement 
needs revision. The investigations of Koch 
on anthrax soon followed, and then came 
the growth of pure cultures of several path- 
ogenic bacteria. 
“The work of Pasteur and Koch afforded 
the first basis on which the study of artifi- 
cial immunity could be again undertaken. 
The possibility of voluntarily producing a 
number of the most important infectious 
diseases of men and animals, and of modi- 
fying at will pure cultivations of bacteria, 
either, according to Jenner’s precedent, by 
passage through the animal body, or other- 
wise on artificial culture media, laid the 
foundation on which advancement could 
proceed. Pasteur himself was the first, 
after Jenner, to produce an artificial im- 
munity by using an attenuated virus; and 
he was also able to introduce the procedure 
to some extent into practice with most 
beneficial results. Still the theoretical ex- 
planation of all these facts lagged far be- 
hind their practical effects. 'The very able 
investigations of Metschnikoff and his 
theory of phagocytosis were, to many in- 
vestigators, inconclusive. ’’ 
Numerous attempts were made to formu- 
late adequate theoretical explanations of 
the accumulated facts concerning the phe- 
nomena of infectious diseases. The fol- 
lowers of Sydenham looked upon the spe- 
cific disease itself as an entity; while Lotze 
and Virchow viewed it asa process. It was 
clear that a mechanical or dynamical proc- 
ess could not be a living entity. The 
