56 THE CAUSATIOx\ OF DISEASE. 



Examples might be multiplied ad lib. to show that all the 

 vital rhythms exhibit a similar tendency to natural variation. 

 In order to make this clear, we must separate in imagination 

 — and this is quite admissible — the tissue, or tissues, especially 

 engaged in the rhythm. Let S stand for such tissue or tissues ; 

 then be it observed that under a given environment, which 

 must include both the external and internal environment, the 

 rhythm will steadily continue to recur, and, if only the material 

 conditions represented by S and E remain the same, the rhythm 

 will recur with undeviating regularity until the end of time. But 

 these material conditions will not remain the same. In the 

 first place, the S is continually altering as life advances — 

 through infancy, childhood, youth, maturity, and old age. 

 In the second place — and it is on this that I wish especially 

 to lay stress — the external-body-environment is no constant 

 factor, and this irregular external-E, reacting upon the S, will 

 tend to modify it, and with it the rhythmical phenomena, 

 which will thus vary; and this alteration, or natural varia- 

 tion, will tend to be inherited by the next rhythm. Thus, to 

 speak only of pathological processes, we know that rhythmical 

 neuralgias and asthmas are apt to vary from one attack to 

 another ; any peculiarity which has been impressed upon the 

 asthmatic paroxysm, by peculiarity of climate or otherwise, will 

 tend to reappear in the next attack. Who that has read 

 Darwin can fail to be struck by the following passages : 

 " If asthma is relegated to unchanging external influences, its 

 cycle of phenomena will go on repeatedly, with a marvellous 

 exactness, but the maintenance of this unvarying repetition is 

 strictly dependent on the maintenance of identical external 

 conditions, and any change, however trifling, is capable of 

 breaking the existing habit, and of introducing fresh pheno- 

 mena."* Again: "Any peculiarity that has once been 

 acquired, (although, perhaps, from some transient accidental 

 circumstance), will, as the time comes round, recur, and thus be 

 formally adopted among the symptoms, and become a constituent 

 part of the clinical phenomena. There are hardly any circum- 

 stances of the disease, that may not thus be lost and acquired. "f 

 * "On Asthma" (Salter), chap. iv. f Ibid. 



