ADJUSTMENT TO INFECTIOUS DISEASE 4O7 



as in human economic relations, the parasite, if the con- 

 dition becomes habitual, pays for the situation by the loss 

 of one or more of the functions no longer needed, a sort of 

 biological degradation. If the cohabitation becomes mutually 

 advantageous, as in the root tubercles of the leguminosae, 

 in the symbiosis of green algae and certain flagellates, or 

 perhaps in the case of colon bacilli and various animals, 

 there is a sort of shrewd metabolic opportunism in which 

 the loss of biologic liberty pays for the comforts of pre- 

 digestion, or in other words civilization. Such perfect, 

 even mutually helpful adaptation, however, is relatively rare 

 and in most instances the entrance of one living unit into 

 the substance of another is either entirely prevented, or 

 resented by the initiation of a struggle, as a result of which 

 one or the other participant is destroyed. Little is known 

 regarding the conditions which ordinarily prevent such 

 invasion except that it is intimately bound up with the 

 property of life and closely associated with the activities of 

 the various enzymes by which the host maintains his 

 metabolic equilibrium and by which the invader attacks the 

 substance of the host. It is not, at any rate, the unsuitability 

 of the environment within the jelly of a mass of frog's eggs 

 which keeps the bacteria in the puddle from swarming into 

 it; for, let a sudden frost kill the eggs over night and, as 

 Bail has pointed out, the mass swarms with invaders before 

 the following morning. In general, then, Hving things, 

 though surrounded by innumerable other living things which 

 could readily make use of their body substances, are pre- 

 served during life from such invasion. 



The majority of microorganisms, of the same classes and 

 orders as those which can cause fatal infection, are eco- 

 nomically independent, living on dead organic and inorganic 

 materials, and delicately adapted to many diff'erent types of 

 environment. A relatively small group have developed the 

 capacity of living in or upon the animal body, and the 

 nasopharynx, the intestines, the conjunctivae and other 

 parts of the accessible body have all developed their charac- 

 teristic flora. It is out of these in most cases that the patho- 

 genic or disease-producing groups have arisen, a process of 

 evolution which it is easier to conceive in the case of bacteria 



