THE INSTINCT OF SURVIVAL 117 



through the skin), but must have an opportunity 

 to reproduce. When we consider for the first time 

 how varied are the injuries that may be inflicted on 

 the body by bacteria, we wonder how any human 

 being manages to run the gantlet of his foes, and 

 defying them all, attain to an old age. During 

 infancy there are the many agents of infection for 

 the digestive tube, leading to the death of enormous 

 numbers of children under five years of age. Then 

 there are the dangers of childhood from diphtheria, 

 scarlet fever, and measles, the causes of the eruptive 

 diseases being still unknown. The period of ado- 

 lescence and early adult life is one of comparative 

 freedom from the inroads of injurious bacteria, for 

 it is a time of growing powers and strong defenses. 

 Still, even at this time, the infections of tuberculosis 

 and of typhoid fever claim many victims, while the 

 entry on sexual life is like opening Pandora's box, 

 in which lie syphilis, gonorrhoea, the dangers of 

 maternity, and a host of more subtle agents of 

 destruction. In middle life the powers of resist- 

 ance are gradually lessened in all directions. The 

 powers favoring decay are more and more successful 

 in finding the weak spots in the armor, and, penetrat- 

 ing a little here and there, make those quiet or more 

 obtrusive inroads on health which point the way of 

 future decline. For it is a fact that by middle life 

 - and often before the tendencies of the tissues 

 have declared themselves and are clearly visible to 

 those who know how to look for them. In one, it 



