96 THE CAUSATION OF DISEASE. 



Wherefore we may say that, in proportion as any particular 

 disease is dangerous to life, and in proportion as it occurs early 

 in life, its chance of passing along several generations becomes 

 less. 



As regards those lesser ills to which flesh is heir, natural 

 selection will play some part. Such of them as depend upon 

 a general weakness of body will tend to be weeded out, through 

 the weeding out due to such general weakness. But there are 

 many local morbid actions quite consistent with thorough vigour 

 of health, and having no power to shorten life. Thus, a family 

 of remarkable bodily vigour is very liable to nasal catarrh. 

 The father and several of the children suffer therefrom, and 

 the tendency can be traced back through several generations. 

 Let me instance more particularly one member of the family — 

 a son, approaching the prime of life. In mind and body he 

 far surpasses the average man, his sole apparent weak- 

 ness, excepting perhaps a slight dash of the nervous tempera- 

 ment, being this tendency to nasal catarrh. Let him expose 

 himself as he will, there is not the slightest inflammation of the 

 throat, bronchi, or lungs, not the faintest rheumatic twinge ; 

 but he is rarely free from some degree of nasal catarrh, and 

 this under exposure becomes acute. The tendency runs back, 

 along the father's side, so long as there is any family record ; 

 but, inasmuch as the ailment is no part of a constitutional state 

 tending to premature death, and is, moreover, strictly local and 

 devoid of danger, it might as such be transmitted ad infinitum,, 

 were it not that the proclivity tends to be diluted at each 

 successive marriage. Let us, however, suppose the catarrh in 

 question to be part of a strumous diathesis, it would then 

 have less chance of transmission, seeing that this diathesis 

 tends to family extinction ; or, let a similar catarrhal tendency 

 affect the larynx in place of the nose, there would, under such 

 circumstances, be no possibility of a long-continued transmis- 

 sion, for it would inevitably kill by occluding the larynx. 

 Thus, natural selection explains the interesting fact that 

 hosts of persons now living have suffered from absolute 

 occlusion of both nares, while there are none, excepting 

 such as have been saved by tracheotomy — and their number 

 is very few — who have suffered from a similar complete occlu- 



