1893. ON PASTEUR'S METHOD OF INOCULATION. 103 
difficult to see that a crop of a given parasite may so far use up a 
constituent existing in small quantities in the body, but essential to 
the growth of the parasite, as to render the body unfit for the 
production of a second crop. The soil is exhausted, and until the 
lost constituent is restored, the body is protected from any further 
attack of the same disorder. Such an explanation of non-recurrent 
diseases naturally presents itself to a thorough believer in the germ 
theory, and such was the solution which, in reply to a question, I ven- 
tured to offer nearly fifteen years ago to an eminent London physician. 
To exhaust a soil, however, a parasite less vigorous and destructive 
than the really virulent one may suffice; and if, after having, by 
means of a feebler organism, exhausted the soil without fatal result, 
the most highly virulent parasite be introduced into the system it will 
prove powerless. This, in the language of the germ theory, is the 
whole secret of vaccination.’’4 
Serious objections may be urged against both explanations. 
Indeed, the difficulties in the theoretical conceptions of how the 
benefits are supposed to arise are sufficiently great even to raise 
doubts as to the reality of the benefits themselves. 
At the very outset of our enquiry, we are confronted by the fact 
that these organisms, which are thus supposed to depend for their 
power of increasing on certain substances secreted in small quantities 
in the animal body, can yet be grown outside of it in various pre- 
parations. The various meat broths, sugar and peptone, extract of 
beef, yeast water, blood serum, &c., which are used, can scarcely be 
supposed to contain these substances, which are not essential consti- 
tuents of the body, since the animal—as in the case of those inoculated 
for any disease—can live and be perfectly healthy without them. 
Whatever, at any rate, may be thought of the others, water of 
yeast, in which Pasteur cultivated the bacillus of splenic fever, cannot 
be supposed to do so; and although boiled potato is a medium not 
generally used for pathogenic—that is, disease-producing—microbes, 
they have been cultivated on it. Thus, Dr. Klein (‘‘ Micro-Organisms 
and Disease,” p. 50) relates that good crops of the micrococcus of 
pneumonia had been reared on boiled potato. Again (p. 103), he tells 
us how Groffky grew the bacillus of malignant cedema on potatoes. 
This, at least, most certainly shows that the growth of the microbe 
cannot depend on a something which exists only in minute quantities, 
and as an unessential ingredient in the blood or tissues of an animal. 
In these preparations, moreover, the microbes increase not merely 
until they might be supposed to exhaust the medium of the small 
quantity of this something, but until they have exhausted it of all its 
nutritive elements, or have produced enough of their toxic principle 
to prevent their own further growth. 
If this something is a normal constituent, how can the blood 
be deprived of it without serious injury to health? If, on the other 
4 Louis Pasteur: his life and labours.” Introduction, pp. xxxv. and xxxvi. 
