NOTES AND MEMORANDA. 651 
by them is almost sure to yield evidence of the presence of 
more than one species, the various species growing together 
in inextricable confusion. On this account it has been 
found a matter of extreme difficulty to determine what 
effects are due to one species of Bacterium and what to 
another. And it has indeed been often impossible to deter- 
mine in such a mixture of forms those which are genetically 
related to one another, and therefore to distinguish one 
species from the other forms which are adventitiously asso- 
ciated with it. 
To effect the separation of species in a mixture, Mr. 
Lister employed a method of dilution and division des- 
cribed in his well-known research on the Lactic ferment 
(see this Journal, vol. xviii, p. 191). Making use of a fluid 
as the nutrient medium of cultivation (as hitherto has been 
the almost universal practice in such cultivations), Mr. Lister 
introduced a drop of sour milk containing possibly twenty 
kinds of Bacteria, and among them the Bacterium of lactic 
fermentation, into a large quantity of pure water, the dilution 
and spacing (so to speak) of the Bacteria thus affected being 
calculated so to render it probable that a séngle drop removed 
from the diluted Bacterian mixture would contain a single 
Bacterium. Such drops were then removed and placed each 
into a separate culture-tube containing sterilized fluid nutri- 
ment, and thus in a certain number of the tubes a pure cul- 
tivation consisting of the progeny of a single Bacterium, 
and, therefore, unquestionably of but one species, was 
obtained. 
This method is tedious and liable to failure owing to the 
great care necessary to ensure and maintain sterilization of 
the cultivation fluid whilst exposed for the purpose of inocu- 
lation and again for further examination. Dr. Koch was 
led to this new method of cultivation, which essentially con- 
sists in the substitution of a solid for a fluid medium of 
cultivation, by the use of the method! known to all my- 
cologists of cultivation, upon slices of potato or beet-root. 
It is readily observed when slices of boiled potato are ex- 
posed in a damp condition to the atmosphere that the sur- 
face of the slice becomes the seat of development of various 
Bacteria and of moulds, the spores of which fall from the 
atmosphere on to the exposed slice, a fact which struck Dr. 
Koch as of importance in reference to the slices of potato 
was this—that the various spores falling on to it remain 
where they fall, and from the spot where each spore or germ 
originally fell it proceeds to multiply, producing around it a 
symmetrical hemispherical growth of perfect purity. In fact, 
