i893. ON PASTEUR'S METHOD OF INOCULATION. 105 



On the other hand, if the disease is caused by the production of 

 some toxic principle, we must surely suppose this power of production 

 to be an essential part of the nature of the microbe, and it is difficult 

 to understand how this power could be lost by mere cultivation ; for 

 the fact is insisted on that the modified virus is the same organism — 

 a definite and distinct species of microbe — as the one which produces 

 the disease in its deadly form ; and if it produces the same poison it 

 ought to produce the same disease. 



If, again, the microbe produces disease by depriving the blood 

 and tissues of matter essential to life, would not the modified cultiva- 

 tion do the same ? In fact, we may assume that to use up the 

 essential specific matter — "exhaust the soil" — the microbe of the 

 modified cultivation would have to increase as greatly as the unmodi- 

 fied form would require to do to produce the disease ; and so, either 

 by obstructing the circulation by mere numbers, by robbing the 

 organs of their nourishment, or by the production of morbific matter, 

 it would cause the same inconvenience as the original virus. 



Leaving these difficulties, what grounds are there for the 

 assumption that there can exist in the blood and tissues a something 

 secreted slowly and in small quantities, and yet not essential to 

 health ? x^s far as I am aware, there is absolutely no proof of the 

 existence of anything of the sort. According to Pasteur it is secreted 

 continuously, and yet in such small quantities that it requires in some 

 cases years to restore a sufficient quantity to the blood to allow of the 

 free growth of the organism. It seems to me contrary to the general 

 principles of physiology to suppose the blood is continuously elaborating 

 a useless product, which accumulates, and yet causes no harm : it is a 

 direct violation of the much-talked-of " law of parsimony " in nature. 



A curious and ingenious suggestion as to what this essential 

 somethingmay be is made by Dr. Maclagen, in an article on " Influenza 

 and Salicin " {Nineteenth Century, February, 1892). 



The microbe, he says, is a parasite, and every parasite requires, 

 besides the essential elements of growth, its own particular nidus. 

 Each microbe finds its nidus in some special part of the body — the 

 liver, the spleen, the skin, &c., and this nidus, he suggests, may be 

 some now-useless character handed down from some of our remote 

 ancestors : 



'* This nidus once exhausted, is, as a rule, never replaced, 

 showing that, like our rudimentary tail, it is something which is not 

 really essential to our well-being — like our rudimentary tail, it may be 

 some peculiarity derived from a very remote ancestor." 



Once exhausted, this nidus is not replaced, and that, according to 

 Dr. Maclagen, accounts for the immunity which one attack of disease 

 confers from a second. 



It does not, however, account for the fact that inoculation 

 requires to be frequently renewed — every year for splenic fever 

 .according to Pasteur, and every seven years for small pox according 



