338 BIOLOGY: GENERAL AND MEDICAL 



stroy the body defenses, toxins by which the cells of the 

 host are poisoned, enzymes by which they are dissolved, 

 and alter the chemical reaction of the tissues in which 

 they arrive. Before the parasite destroying mechanism 

 can be set in operation, therefore, something in the way 

 of endurance, tolerance, or immunity against the micro- 

 parasitic products becomes essential. The reactions of 

 immunity, therefore, become exceedingly complex. They 

 embrace two essentials ability to endure the microorgan- 

 ismal products without injury and ability to destroy the 

 microparasites. 



Before further pursuing the subject of infection, a 

 brief space must be devoted to general considerations 

 that bear upon the subject of immunity. 



Habit has much to do with the general vital processes. 

 It is exemplified by the change of animals from the 

 independent and synthetic to the dependent and ana- 

 lytic mode of nutrition; by the remarkable differences in 

 the behavior of aquatic animals to salt and fresh water. 

 The great marine mammals, the fishes, the numerous 

 phyla of marine invertebrates, and large numbers of 

 marine birds find sea-water with its heavy percentage 

 of sodium chloride and other salts unobjectionable; 

 other animals and plants habituated to fresh water are 

 poisoned by it; still other animals and plants living in 

 the mouths of rivers endure alternating immersion in 

 salt and fresh water resulting from tides and currents 

 without injury. The precisely arranged adaptations 

 under which living things present themselves to obser- 

 vation at the present time are the result of divergences 

 begun long ago, and fail to suggest the tremendous 

 sacrifice of life though which they have probably been 

 adjusted. 



Every creature is found upon examination not only 

 to consume by preference some particular kind of food, 

 but to find radical departure from it fatal and even 

 slight departures harmful. Even where the diet appears 

 to be of mixed nature it is limited to a few, not to all 



