THE CAUSATION OF DISEASE. 95 



generations, become so structurally welded with other parts that 

 they are with as great difficulty dropped from the race as is 

 "granular kidney" from the individual; for. all parts of the 

 body being more or less interdependent, a variation in one 

 part necessarily leads to alterations in other parts, these again 

 to others, and so on ; and thus, in the course, of ages, the 

 initial variations may become so ingrained as to be incapable 

 of suddenly vanishing; and this seems to afford a strong 

 argument in favour of the proposition that mere repetition must 

 render a character more permanent. I suspect that whenever 

 traits of great ancestral age are rapidly dropped under 

 changed conditions of life, they are of a so-to-speak superficial 

 character, not largely influencing or being influenced by other 

 tissues ; and thus, not being intricately interwoven with the 

 rest of the body-fabric, they are detached with comparative 

 ease. 



It is very important to note that the ancestral age of any 

 particular pathological variation — that is to say, the number of 

 afflicted generations — is usually not great. In thus speaking 

 of the ancestral age of any particular disease, it will be observed 

 that I refer, not to the date of its first appearance in the human 

 race, but, as I say, to the number of successive generations 

 ■of the same family labouring, actually or potentially, under 

 that disease. 



What are the grounds for thus asserting that the ancestral 

 age of disease is short ? This conclusion follows from the 

 doctrine of natural selection. Through untold ages there has 

 been a survival of the fittest, such survival leading to the 

 evolution of organisms structurally adapted to their respective 

 environments. Now, an individual afflicted with disease — that 

 is, a pathological variation — is not thus adapted, and tends, in 

 consequence, to be weeded out. Thus it happens that a serious 

 disease would have little chance of being handed down through 

 many successive generations of one family, for ultimately extinc- 

 tion must ensue. These remarks do not so forcibly apply to the 

 lesser ailments which seem to be a necessary accompaniment 

 of animal life, nor do they apply in any way to diseases which 

 only appear after the procreative period of life, since these can 

 in no way affect the offspring : but such diseases are very rare. 



