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. "+ 



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, 

 the'y 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 Groftky grew the bacillus of malignant oedema 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 

 ' " Louis Pasteur : his life and labours." Introduction, pp. xxxv. and xxxvi. 



