1893. 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? As 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 tosuppose the blood is continuously elaborating 
a useless product, which accumulates, and yet causes no harm: itisa 
direct violation of the much-talked-of ‘‘ law of parsimony ” in nature. 
A curious and ingenious suggestion as to what this essential 
something may be is made by Dr. Maclagen, inan 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 
