io6 NATURAL SCIENCE. Feb., 



subsequent protection ; and the attempt was in a measure successful. 

 The substitution of vaccination for inoculation was, at its inception, 

 a purely empirical step, since at that time the identity of cowpox and 

 smallpox was only a surmise. But in the light of recent knowledge 

 vaccination receives its full scientific justification, as being a pro- 

 tective inoculation with a virus attenuated by transmission through a 

 relatively insusceptible animal. 



The discovery of the bacterial nature of infective diseases was a 

 necessary preliminary to the right understanding of the true nature of 

 immunity, and to Pasteur belongs the credit of first deliberately 

 attenuating the virus of a specific disease with a view to protective 

 inoculation, i.e., to the production of a mild and non-fatal attack of 

 the disease, which should confer subsequent immunity. Modern 

 bacteriology has pressed chemistry into its service, and our knowledge 

 of the mode of action of at least some pathogenic organisms in pro- 

 ducing disease is tolerably clear. 



Various explanations have been advanced of the essential nature 

 of susceptibility and immunity. They fall into two groups, the 

 chemical and the vital, and the truth probably lies between the two. 

 To the vital group belong the doctrine of phagocytosis, advanced by 

 Metschnikoff, according to which the leucocytes of the body are 

 charged with the duty of destroying micro-organisms which gain 

 access to it, and also Sternberg's theory of acquired tolerance to the 

 toxic products of bacteria on the part of the cells of the body. It may 

 be admitted that leucocytes possess the power of swallowing foreign 

 bodies, such as bacteria, and they probably swallow them alive as well 

 as dead ; but Metschnikoff's attempt to represent the leucocytes of 

 the body as a sort of local police, on the success or failure of which 

 to cope with an invading army of micro-organisms immunity or 

 susceptibility depends, is not a complete and satisfactory explanation of 

 all the observed facts. Valid objections were soon urged, also, against 

 some of the earlier chemical theories of immunity : such were the 

 " exhaustion " theory, once advocated by Pasteur, according to which 

 a given micro-organism, having once flourished in an animal body, 

 used up some pabulum necessary to its growth and never subse- 

 quently renewed, and the " retention " theory of Chauveau, which 

 postulated the retention in the animal body, as the result of bacterial 

 growth there, of some substance produced by the bacteria, and 

 inimical to their own further development. The view which has 

 gradually gained ground as affording the best explanation of observed 

 facts is the " antitoxin " theory, which is at once a chemical and a vital 

 conception. It supposes that the animal body has the power of 

 forming some substance either directly germicidal or capable of 

 neutralising the toxic products of bacterial activity. The natural 

 presence of such a substance gives immunity; its absence leads to 

 susceptibility; its development, as the result of an attack of the disease, 

 represents acquired immunity. 



