THE CAUSATION OF DISEASE. I 1 9 



The question now arises, may reversion occur to a condition 

 which from the beginning has been pathological ? — may, in other 

 words, an individual inherit a disease from a remote ancestor — 

 e.g., gout, rheumatism, or insanity ? Diseases are, as we have 

 seen, pathological variations ; and why, it may be asked, should 

 not an individual tend to revert to a pathological t quite as much 

 as to a physiological, state ? A little consideration will show that 

 the tendency to revert to the diseased state of a remote ancestor 

 is infinitely less than to revert towards a physiological state. 

 We saw that a pathological variation must necessarily have a 

 much looser hold upon a race than one that is physiological, for 

 such variations are, owing to a survival of the fittest, being 

 continually weeded out. Total extinction inevitably prevents 

 them from being handed along many generations of a particular 

 family line ; therefore they never leave a deep impress upon 

 the organism, and, after they have been dropped for a few gene- 

 rations, we may expect this impress to have completely faded 

 away. "We saw that certain minor ills that do not endanger 

 life may obtain a firmer hold, and therefore we should expect a 

 greater tendency towards a reversion to such. Eeversions to 

 pathological states belonging to a few generations back may, of 

 course, occur; but it is highly improbable — indeed I should 

 say absolutely impossible — for them to occur to a patho- 

 logical state of such far-off ancestral date as may happen in 

 the case of reversions to physiological states. The stripes of 

 the mule are derived from a very remote ancestor, but then 

 they were a constant ancestral character during many thousands 

 of generations, and made a correspondingly deep impress upon 

 the organism and its embryological processes. No pathological 

 character could acquire such a hold. These considerations 

 should tend to dispose of the theory that many sporadic diseases 

 are reversions to such remote pathological states. (It has been 

 suggested, for instance, that leprosy sporadically occurring in 

 non-leprous countries has such an origin.) 



Nevertheless, I see theoretical grounds for assuming that 

 disease may result from reversion to a remote ancestral state. 

 Thus, the tissues may revert to remote ancestral states, which, 

 though perfectly physiological in themselves, may be injuriously 

 susceptible to influences which are incapable of hurting the 



