136 AN INTRODUCTION TO SCIENCE 



out the great mission of bacteria in returning all organized 

 beings to the soil. 



In the struggle with its invaders the organism wins in the 

 long run ; but myriads of individuals die before immunity to 

 any form of disease is acquired by the race. The progress 

 which is being made towards the acquisition of a power of 

 resisting disease is strikingly shown in the innocence of 

 measles among the white races compared with their viru- 

 lence when introduced into the South Sea Islands and other 

 places where they were previously unknown. No diseases 

 are restricted to definite geographical areas in these days of 

 free communication ; but there is evidence that even within 

 historic times the evolution of human beings has tended to 

 protect them against those forms of germ to which they 

 were especially exposed, while the evolution of the germs 

 has resulted in the production of new forms of disease. 



The animal body counteracts the toxins which microbes 

 produce by developing within its tissues and juices a class 

 of substances to which the collective name of antitoxins has 

 been given. Take diphtheria as an example. When a 

 susceptible animal, such as the horse, is inoculated with the 

 diphtheria toxin it exhibits symptoms of the disease. If the 

 dose is small the horse recovers. After an interval (of say 

 five days) a larger dose of the toxin is required to produce 

 disturbance. Eventually, an unlimited dose may be given 

 without effect. It is immune because its blood is charged 

 with antitoxin. And now, if some of its blood-serum, 

 which has been perfectly sterilized so that it will keep for 

 weeks or months, is injected beneath the skin of a child 

 suffering from diphtheria, the antitoxins of the horse rein- 

 force those which the child is making for itself and enable 

 it, if the case has not advanced too far, to antidote the 

 toxins of the disease. 



Vaccination confers immunity in a somewhat different 

 way. For some reason, which is not as yet satisfactorily 

 explained, after ooe invasion of a particular kind of germ 



