THE CAUSATION OF DISEASE. 97 



sion of the larynx. Natural selection, alike merciful and 

 merciless, prevents the inheritance of such a fatal tendency, 

 by yearly sacrificing thousands. From these examples we see 

 how fatal disorders, occurring before the period of procreative 

 life, cannot well pass through many generations, while such as 

 are not fatal may attain considerable ancestral age. 



We, may, therefore, conclude that any given instance of 

 serious pathological variation is of comparatively recent ancestral 

 date : whence it follows, from the principle just now enunciated 

 (namely, that the fixity of a variation depends in large 

 measure upon its age), that pathological variations are for the 

 most part unstable. In other words, a diseased state shows a 

 great teudency to revert to the status quo ante — viz., health. 

 This tendency shows itself in a twofold way : first, in the 

 tendency of a permanently diseased state in the parent 

 not to appear in the offspring ; for, although disease is un- 

 doubtedly hereditaiy, yet we see " Nature " ever making a strong 

 effort to throw it off. (This is best shown where man and wife 

 have very many children : speaking generally, we shall find 

 that only a small minority will inherit the disease. It is in 

 such large families, indeed, that heredity in disease can best be 

 studied.) Secondly, the tendency to fling off a disease is shown 

 in an individual who has himself acquired it. Instance the 

 several acute disorders, such as pneumonia and nephritis. It 

 is, of course, not to be supposed that the pathological acquisi- 

 tion can be thrown of — that there can be reversion to the status 

 quo ante — where there is a gross organic lesion, such as a 

 mutilated heart valve or a fibrotic kidney ; but the truth of the 

 principle is shown in the tendency of these parts to be healthy 

 in the offspring. 



Disease must, then, be regarded as a more or less temporary 

 acquisition, tending under a proper E to disappear. This 

 tendency shown by diseased tissues to revert to their ante- 

 morbid state is learnedly called the vis medicatrix natural* 

 I find that Paget, if I understand him rightly, explained this 

 vis medicatrix in a somewhat similar way.f 



* H. Spencer biings the etf medicatrix naturce under his principle of equili- 

 bration. See his " First Principles," chapter xxii. 



f " It seems probable that in embryo life there is, generally, a tendency to 



H 



