May, 1889 
PRESIDENT S ADDRESS 
107 
confluent with other colonies, we shall be quite sure that a 
minute particle, taken from one of these colonies, contains 
the products of only a single germ. That this assumption is 
justified by the facts can easily be shown. It is possible in 
this way to take any given species that will thrive under the 
conditions, and propagate it successively from plate to plate 
for any number of generations, always maintaining the same 
character (or the same combination of characters) both in the 
macroscopic and in the microscopic aspects of the colonies. 
There cannot remain the slightest loophole for doubt that 
we have obtained the required species “ pure ” and free from 
all admixture ; moreover, we are able to rebut another 
objection that was frequently made to the old experiments, 
namely, that the cause of death where an animal was 
inoculated with Bacteria was not the organism itself, but 
some chemical substance which accompanied it. For, since 
the chemical substance being inorganic would be incapable 
of increase, while the Bacterium would multiply itself to any 
extent required, it is obvious that the ultimate result, after 
many successive cultivations, would be the entire elimination 
of the hypothetical chemical compound, and the production 
of a material in which the only element present, except the 
nutrient medium, was the organism under investigation. 
It is of course necessary that, in all the manipulations 
required, precautions should be taken to exclude all foreign 
germs, but the methods of sterilising (as it is called) every¬ 
thing that is used—gelatine, flasks, plates, needles, etc.—by 
means of heat or steam or acid, are now so well understood 
and so successful, that no danger need be feared on this 
score. Moreover, we can use that principle, which is now so 
widely and so constantly employed, of a “control” experi¬ 
ment. For if we go through exactly the same manipulations 
with two portions of the material, but sow germs on the one 
but not on the other, and if we invariably see the one on 
which nothing was sown remain unchanged, while the other 
reproduces the organism with which we are experimenting, 
then we are entitled to conclude that our precautions for the 
exclusion of foreign germs have been entirely satisfactory. 
We are now in a position to investigate the morphology and 
biology of these minute organisms with the same certainty 
with which we can experiment with the seeds of Phanerogams. 
One of the most curious practical applications of this 
principle of “ pure ” cultures is found, not indeed among the 
Bacteria, but in a group closely allied to them. One of the 
chief functions of Bacteria in the world is that of inducing 
fermentation ; and their role in this respect is shared by the 
