Abstract (i) Variable Happening.—In Section IX, we commenced by touching upon the necessity of studying the effect of changes in the happening-element, but there dealt only with the case of hypometric happening. We now proceed to examine other changes in this element. It is unlikely that in any infectious disease the infectivity-element c will always remain constant. It may remain practically such for some length of time; and the hypothetical results of this condition have therefore been analysed above in detail; but nature abhors a straight line, and we may infer from general experience that changes are sure to occur from time to time. As suggested in Section I, changes in infectivity may be due— (a) to the action, so to speak, of the infecting organisms themselves—which is difficult to believe in because it involves the conception that organisms living in large numbers of different hosts should, as it were, make a kind of concerted and simultaneous effort; and (b) to changes in season, climate, or other environment which may (1) increase or diminish the infective strength of the infecting organisms; or (2) facilitate or hinder their transference from one host to another.