696 
(b) Ina virulent condition, in non-lethal doses. 
2. By injection of the dead organisms. 
3. By injection of filtered bacterial cultures, 7. e., tox- 
ines; or of chemical substances derived from these. 
These methods may also be combined in various 
ways. 
B. Passive immunity, 7. e., produced in one animal 
by injection of the serum of another animal 
highly immunized by the methods of A. 
1. By antitoxie serum, 7. e., the serum of an animal 
highly immunized against a particular toxine. 
2. By antimicrobic serum, 7. e., the serum of an ani- 
mal highly immunized against a particular or- 
ganism in the living and virulent condition. 
The protective value of active immunity 
extends through a considerable period of 
time, while that of passive immunity is 
evanescent. 
‘An adequate explanation of this vast 
array of facts is yet before us. The ex- 
planation in detail cannot be given to-night ; 
that must await another time; but some 
generalizations must be made. 
1. Pasteur’s theory of exhaustion of the 
pabulum is disproved by the fact of passive 
immunity. 
2. The theory of retention will have to be 
greatly modified before it can explain many 
facts with which it is now in opposition. 
3. The theory of acclimatization or habitua- 
tion has limited application and, like the 
theory of adaptation, takes too little cog- 
nizance of details. 
4, Metchnikoff’s theory of phagocytosis falls 
before the facts of passive immunity ; and 
5. The humeral theory only presents an- 
other phase of its own evolution. 
6. Buchner’s hypothesis, which explains 
immunity as being due to the reactive 
changes in the integral cells of the body re- 
sulting from the action of chemical products 
absorbed from the seat of vaccination or in- 
oculation, is strongly supported by experi- 
mental evidence ; and 
7. Ehrlich’s side-chain (Seitenkette) theory 
presents an exceedingly ingenious and in- 
teresting explanation of the phenomena of 
SCIENCE. 
(N.S. Vou. XIII. No. 331. 
immunity adduced by experiments in vitro 
and in vivo. 
By elimination the problem may be some- 
what simplified. The facts themselves may 
be roughly divided into two groups: (1) 
biological, and (2) chemical; and the ex- 
planations will then be either biological or 
chemical. In the ultimate analysis, the 
biological explanation will rapidly pass 
from the body as a whole to its respective 
organs and their respective cells, to the 
nucleated cells, and finally to the biogen of 
the nucleus; while the chemical explana- 
tion will describe the cycle that begins with 
the minutest atomic reaction, passes on- 
ward through more and more complex intra- 
and intermolecular synthetic and analytic 
changes so long as chemical equilibrium is 
disturbed ; but eventually finds its begin- 
ning and its end—cause and effect—in 
energy potential, energy kinetic, liberating 
impulse. 
That the problem of immunity will be 
solved is only a question of time. The 
active research now in progress is rapidly 
dissipating the unknown; and when the 
chemical structure of the various animal 
proteids becomes a known quantity their 
interaction will be readily seen and the so- 
lution of the problem will be an accom- 
plished fact. 
The problem is a biochemical one, and 
biochemists will solve it. Many, if notall, 
the phenomena of fermentation, infection 
and immunity are explainable in terms of 
modern chemistry, and since modern chem- 
istry is firmly founded on the doctrine of 
energy we have to consider merely the 
terms, energy potential, energy kinetic and 
liberating impulse. 
I am conscious of having failed to bring 
before you a large mass of newly accumu- 
lated, interesting facts which should be 
considered in this connection; but the 
largeness of the subject together with the 
enormous accretions annually made to its 
