260 ROYAL SOCIETY OF CANADA 



probably due to the fact that for ages they have been attacked by these 

 diseases and their tissues have in time become acclimatized, as it were, 

 to the deleterious influences which are brought to bear upon them. In 

 v/hat this increasing resisting power essentially consists we are not in 

 a position to say, but it is clearly inherited. The susceptibility of 

 certain families to diseases like tuberculosis is probably also a heredit- 

 ary peculiarity, the tendency being as it were, focussed in such per- 

 sons as a result of certain accidents of marriage. No satisfactory 

 explanation of personal idiosyncrasies has ever been offered. 



We pass on now, however, to a more fruitful field for enquiry, 

 namely, acquired immunity. It is a well known fact that individuals 

 can acquire a toleration of poisonous substances by the ingestion of 

 these substances in gradually increasing doses over prolonged periods 

 of time. The chronic alcoholic, the morphinomaniac, and the arsenic- 

 eaters of Styria are cases in point. Similarly, one attack of an infec- 

 tious disease will in many cases confer a more or less lasting immun- 

 ity. Small-pox, scarlatina, t^-^ohoid fever, and syphilis, rarely attack 

 the same individual more than once, though exceptions to this rule 

 undoubtedly occur. The degree of immunity in such cases varies 

 greatly with the disease and the individual. The immunity after syph- 

 ilis and small-pox is almost absolute, after other infectious diseases 

 is less marked. Immunity of this kind, active immunity as it is called, 

 can also be produced artificially. 



Perhaps the earliest example of this of which we have authentic 

 information is the inoculation for small-pox practised in the East and 

 referred to by Lady Mary "VVortley Montague in her letters written in 

 the earlier part of the eighteenth century (1718). The method con- 

 sisted in the inoculation under the skin of healthy individuals of ma- 

 terial taken from the pocks. In this way a mild form of the disease 

 was in many cases induced which protected against subsequent attacks. 

 The procedure does not seem to have been at all certain in its results, 

 however, and it was not until somewhat later, when Jenner introduced 

 the practice of vaccination with the lympli taken from cases of cow- 

 pox, that reliable results were attained. It is now believed tiiat vac- 

 cinia or cow-pox is small-pox in an attenuated form, and that by giving 

 a person an attack so mild as to be in most cases devoid of more than 

 trifling inconvenience we protect against a much more serious type of 

 the malady. In this case we are probably dealing with an animal 

 parasite, but the same principle applies to certain vegetable parasites, 

 namely, bacteria. It has been found possible to produce inrmunity 

 by the inoculation of living bacteria whose virulence has been attenuated 

 in various ways, as Pasteur did with chicken-cholera and anthrax in 



