504 Chapter XV 



very intimate relation with its practical application. It is evident 

 that the road is far from traversed to its terminus, for there are 

 many infective diseases in which vaccinations cannot be employed, 

 but it is none the less certain that the path which has led to so 

 many important and useful results should still be followed in studying 

 problems which up to the present we have been unable to solve. 



