164 PRESIDENT’S ADDRESS—SECTION H. 
practically, it is the accidental conjunction between specific causes 
and susceptible organisms that we seek to prevent. We may, 
therefore, speak as justly of vaccinating, or of isolating a case of 
fever, to avoid accident—that is to say, to prevent communica- 
tion of the cause of disease to persons unaware either of its 
propinquity or of its quality—as we speak of fencing a revolving 
shaft. It is thus inevitable that we should endeavour to prevent 
some diseases ; all those, namely, which have been shown or in 
the future may be shown to be preventable. 
And such endeavour may be successful. Experience in 
England is taken to show so much. By abatement of some 
diseases a large number of deaths have in that country been 
saved to persons chiefly between the ages of five and thirty-five 
years ; and the average duration of life has been extended for 
persons by as much as two years and a tenth. Thus the 
diminution in deaths has brought about a numerical increase in 
population. It may be contended that, by so many lives saved, 
or by so inany years of life added, the productive power of the 
nation has been increased. For reasons that will appear imme- 
diately, it seems most likely that it has been increased, although 
not to the full indicated extent ; but it has been questioned 
whether such proximate increases will ultimately prove the 
substantial advantage which at first sight they seem to be. It 
has been said that disease attacks the weak, that by checking it 
many of the weak who would have died must be preserved, that 
these will survive to the reproductive ages, and will then either 
reproduce their lke by intermarriage or, by union with the 
strong, will at last lower the general standard of vitality, so 
that the productive power of the nation will be diminished, and 
its tendency become towards extinction. And perhaps the 
following illustration might have been adduced in support of 
that view :—It might perhaps have been suggested that the 
natural terin of life is still set at three score years and ten, and 
four score years is still regarded as an extension too seldom 
enjoyable to be generally coveted, just because disease has been 
left unrestrained during the last four thousand years, has steadily 
weeded out the weaker, and has left the strong to reproduce 
their like. In fact, it seems that there is at bottom much of 
highest importance in the view mentioned ; but it is surely not 
the whole truth. In the first place, the diseases whose abatement 
has led to the saving of life in England which is represented by 
an increase in the average duration of life of two years and a 
tenth, are chiefly the specific contagious and the filth-diseases ; 
and these are the diseases against which (in the main) preventive 
medicine is thus far engaged. It is not to these alone, however, 
that those who are of imperfect constitution succumb. If they 
escape these they still have other chances, of which some important 
ones are inherent to them, of extinction. But, secondly, the 
