130 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



excluded. It is impossible to avoid introducing 

 into the digestive tract many bacteria which would 

 prove undesirable permanent tenants, and these 

 are successfully discouraged from gaining a foothold 

 by the motile biological activities of the more per- 

 manent and better adapted bacterial guests. But 

 to maintain this supremacy, the normal flora depend 

 upon certain conditions of food, on the one hand, 

 and of secretions from the epithelium and digestive 

 glands, on the other. The secretions may be so 

 altered by nervous conditions, or by actual disease 

 of the epithelium, that the preempting flora can no 

 longer maintain themselves. In fact, they gradually 

 (or sometimes rapidly) suffer displacement to a 

 considerable extent by microorganisms capable of 

 leading a life injurious to the secretory structures of 

 the digestive tract and to the organism as a whole. 

 M. Metchnikoff has long maintained that the re- 

 placement of the normal bacterial flora by "wild" 

 intrusive races is a potent cause of chronic disease, 

 and of curtailment of life, and I have long held and 

 taught similar views based on wholly independent 

 study. We have still very much to learn regarding 

 the details of this unique partnership between the 

 human body and the bacterial parasites which 

 inhabit it with advantage to host and to guest, but 

 it is quite safe to make the far-reaching generaliza- 

 tion that all influences which aid in maintaining a 

 normal bacterial flora are factors in the avoidance of 

 disease and in the maintenance of long life, Strenu- 



