166 ON VAEIABILITY AND ADAPTATION 



so injected are, later, to be gained from the various organs of 

 the apparently healthy animals. The tissues are potentially 

 sterile ; that is, the leucocytes and other phagocytic cells are, 

 throughout life, engaged in destroying bacteria which have 

 gained occasional entrance into the tissues. 1 When a few 

 intensely virulent bacteria gain this entrance, or a large number 

 of a less virulent form manage to grow in some one or other 

 locality, and thus set up inflammatory changes, the above- 

 mentioned reserve force in the phagocytic cells comes into play 

 and permits these cells to take up and digest the greater numbers 

 or the more toxic forms. It is this circulation of potentially 

 pathogenic microbes that explains the cryptogenic infections of 

 internal organs. 



2. The second law is that of aceustomance. A cell not 

 actually destroyed by any deleterious agency is apt to become 

 accustomed to the presence and action of that agency. What is 

 the basis of this accustomance it is difficult to say, though it is 

 not difficult to suggest a hypothesis or hypotheses. Here I 

 simply state that this is an observed law, a law best exemplified 

 in what has been made out regarding the conversion of a negative 

 into a positive chemiotaxis. 



3. The third and very important law, somewhat closely allied 

 to the last, is that of habit, or, as Eraser Harris 2 has termed 

 it, " vital inertia." According to this law, when once, through 

 a given stimulus, a certain series of molecular changes is set up 

 in a cell, those changes are liable to continue after the stimulus 

 has ceased, and, if the stimulus be sufficiently strong or sufficiently 

 often repeated, the cell acquires the habit, or property, of setting 

 in action a particular series of molecular changes after a minimal 

 stimulation. This is, perhaps, best exemplified in connexion 

 with our subject in the production of antitoxins. It is found 

 that, once the diphtheritic toxin, for instance, has stimulated 

 the cells of the organism to produce antitoxins, that production 

 continues and is wholly out of proportion to the amount of toxin 

 introduced ; while, similarly, once an animal has gained full im- 

 munity against any micro-organism, it only needs the introduction 

 of that micro-organism into the system to induce an immediate 

 reaction, whereas previously days or weeks had been required 



1 Adami, Journ. Amer. Med. Assoc., Dec. 23, 1899. 

 1 D. F. Harris, Brit. Med. Journ., 1900, ii. 741. 



