I I 8 THE CAUSATION OF DISEASE. 



an individual is potentially endowed with qualities whereof he 

 and his friends are wholly ignorant. Undeveloped heroes — 

 " village Hampdens" — are, perhaps, more common than most 

 people imagine ; certain it is that great crises call into pro- 

 minence men who would otherwise have passed their lives 

 in obscurity, and some historians would perhaps class Cromwell 

 among their number. It often falls to the duty of the physi- 

 cian to develop potentialities by this process of evolution ; for 

 he is not infrequently able to render an individual proof 

 against some particular disorder by favouring the evolution of 

 certain tissue or tissues — such as the tissues whereof the ner- 

 vous system is built. 



4. Finally, a potentiality may be converted into an actuality 

 by disease, atrophy, or removal of the ovaries or testicles, for, 

 under these circumstances, each sex tends to take on certain of 

 the proper sexual characters of the opposite sex. Patholo- 

 gically this fact has its importance, for, when the ovaries 

 remain long inactive, the woman tends to become masculine — 

 probably owing to an atrophy of these organs from disuse, 

 and this more notably occurs after the menopause. In so far 

 as sex is capable of colouring disease, we shall accordingly find 

 these modifications in the sexual characteristics giving a corre- 

 sponding colour to the diseases from which the individual thus 

 sexually altered suffers. 



Let us now consider what further bearing the principle of 

 reversion has upon pathology. In treating of the fixity of 

 structural characters, we saw that the vis medicatrix rested 

 upon the principle of reversion. But this principle has further 

 application in pathology. In the first place, it is manifest 

 that a reversion may be both pathological and physiological. 

 The latter variety is the most common. As an example of such 

 physiological variation we may again instance the reversion in a 

 fancy pigeon to the blue plumage of its far-off ancestor, the 

 Rock pigeon. We have seen, however, that reversion may 

 occur to a condition which, though normal to the ancestor 

 from whom it is derived, is abnormal to the individual in whom 

 it reverts, as when a branchial cleft persists in a mammal. 

 The latter then is, in one sense, a pathological reversion. 



