46 
Proceedings of Royal Society of Edinburgh. [sess. 
recently reported that he found that the organisms which, when 
together, destroyed cholera, did not exert nearly the same effect 
when seeded as pure cultures alone with cholera, a result which he 
seemed to regard as very anomalous. This, however, becomes easily 
understood if we consider the course of events in putrefaction as it 
occurs normally in nature. If we examine from time to time a 
fluid undergoing this change, we find that at different periods of the 
process different organisms predominate, the others, for the time, 
sinking into the hack-ground or even disappearing ; yet, as Hauser 
has shown, several of these organisms can, when grown alone as pure 
cultures, carry out, although much more slowly, the greater part of the 
process. In nature we have, however, a physiological division of 
labour, each organism becoming adapted to a special stage of the 
process, during which hy its more active growth and the molecular 
changes it sets up in the medium, it seems to exert a certain inhibi- 
tive action on the other microbes present. This is more clearly 
shown hy the remarkable way in which yeast* remains essentially 
pure when grown under favourable conditions, although exposed to 
every chance impurity. If the wort is held at a suitable tempera- 
ture, and sufficient yeast is at the first added, so that it may increase 
sufficiently rapidly to get at an early period “ within striking distance” 
of any chance intruders, no other organism appears able to obtain a 
footing, unless when the process is allowed to proceed so far that the 
products of the yeast begin to act depressingly on its own cells. 
The inhibitory action which one microbe appears to exert upon 
another during the vigorous exercise of its vital functions, is strictly 
comparable to that which the tissues may exert upon such organ- 
isms. The cells of the human body have undergone great differ- 
entiation morphologically and physiologically, fitting them to carry 
out the various stages in the dissociations and oxidations necessary 
in the combustion of albumen to urea, carbonic acid, and water. A 
microbe on entering the tissues finds itself in conflict with cells 
specially adapted to the conditions present there, and more or less 
vigorously carrying out their stages in the vital processes, so that it 
has small chance of invading the organism, or even holding its 
ground, unless it exerts a depressing influence by means of its pro- 
ducts — unless it is pathogenic. That we are not here ascribing too 
* Naegeli, Theorie der Gdhrung, Miinclien, 1879. 
