258 ROYAL SOCIETY OF CANADA 



invade the human system. Again, tlie bacillus of anthrax is virulent 

 for guinea-pigs and mice, but has no effect upon white rats. Ordinary 

 sheep are also very susceptible to anthrax, but the Algerian breed are 

 much more résistent. The field-mouse is susceptible to glanders, while 

 the white mouse is immune. The microbe of rabbit septicsemia is 

 excessively poisonous for -rabbits but does not affect rats, guinea-pigs, 

 or dogs. With regard to human beings different races exhibit similar 

 notable differences. Whites, for example, are more susceptible to mala- 

 ria and yellow fever than are negroes. 



Family peculiarities have an important bearing on the etiology 

 of infective disease, as hardly needs to be mentioned. That certain 

 families are particularly liable to tuberculosis, and that when they have 

 contracted it the affection goes badly with them, is a matter of common 

 knowledge. 



We may, moreover, narrow the matter down still farther, namely, to 

 the individual. Personal idiosyncrasy is an element that always has to 

 be taken into account by physicians. Some individuals are abnormally 

 sensitive to the action of drugs, such as quinin, antipyrin, mercury, 

 arsenic, salicylic acid, morphin, iodoform, and the venom of poison-ivy. 



Age is also to be considered. It is a matter of common observ- 

 ation that children are much more susceptible to certain infective dis- 

 ease, such as measles, scarlatina, whooping-cough, chicken-pox, than 

 are adults. On the other hand, children are believed to tolerate large 

 doses of arsenic, belladonna, quinin, digitalis, potassium iodide, and 

 mercury. Diseases like tuberculosis, typhoid fever, and osteomyelitis, 

 are much more common in youth and early adult life than they are in 

 those advanced in years. 



When we come to seek the explanation of the occurrence of natural 

 immunity we are confronted with many and great difficulties. The 

 problem is most complex as most vital processes usually are. It would 

 be idle, therefore, to expect a simple solution or even a single solution. 

 The subject, too, is bound up inextricably with that most abstruse 

 question — heredity. 



By way of clearing the ground for a proper understanding of tlie 

 matters we have to discuss, it is necessary to refer briefly to the means 

 of protection which the animal organism naturally possesses. The 

 ectodermic covering of the body is differentiated into two portions, 

 the skin or epidermis and the mucous membranes. These constitute 

 the first line of defence. Before systemic effects can be produced it 

 13 necessary for the offending agents to pass through the protecting 

 envelope. This is ordinarily effected through the medium of an abra- 

 sion or other solution of continuity. \\Tien once an entrance has 



