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Smallpox and Vaccination 
in view of the fact just mentioned seems to establish that this phenomenon is not 
due to alteration in the character of the population, but presents a biological as 
distinct from a statistical change. The explanations which can be offered from 
the biological point of view are three. Firstly, there may be some secular change 
taking place in the susceptibility of the population towards smallpox itself; 
secondly, there may be a like change taking place in the smallpox organism ; and 
thirdly, it is possible that the passage of the organism of vaccinia through several 
thousand generations in the human subject has gradually increased the protective 
value of lymph ; here again, however, there may be secular change. This increase 
in protective value, while it cannot be held proved, is in striking contradiction to 
what has been hitherto considered as probable. Plants propagated by cuttings, 
by tubers, and such like means, are well known to deteriorate in quality, and 
frequent recourse must be had to propagation by seeds if the strength of the 
species is to be maintained. To this rule, however, there are certain exceptions. 
Though it cannot be distinctly averred, however, tliat the vaccinal lymph has 
increased in potency, it can with all certainty be maintained that there is no 
evidence in favour of its deterioration. The evidence on which the argument 
regarding the deterioration of lymph was based cannot be regarded as complete. 
The chief points were, firstly, that as the century went on it became increasingly 
more possible to revaccinate successfully ; and secondly, that in making attenuated 
forms of sheep-pox corresponding, as was thought, to vaccination, it was found 
that after a few generations these could not be relied on to give protection. In 
view of our fuller knowledge of the probable life history of the smallpox organism 
and its relations to vaccinia this latter argument ceases to present a reasonable 
analogy, while the former is more explicable on the view of the increasing potency 
of the lymph than on the other hypothesis. 
The change just described is one which affects vaccinated persons as a whole 
and bears no special relation.ship to the question of the growth and decay of 
immunity. It is then to be noted that in all further discussions of smallpox 
statistics throughout the course of this paper the comparisons which are drawn, as 
far as possible concern persons in the same epidemic in the same place, because 
with different age constants at different places and at different times it can be 
seen what contradictory inferences could be made. 
The first fact with regard to the immunity conferred by vaccination is one 
which seems unexpected. It is found that persons who are well protected have 
for all epidemics at all places a lower mean age than those who are badly 
protected. Exactly the same remarks apply to the groups which are based on 
the size or character of the cicatrix. Thus in each epidemic persons with four 
good marks have a lower mean age than those with four indifferent marks, a 
relationship which also applies to the group of persons presenting one, two, or 
three cicatrices. Taking the statistics of the large epidemics in Glasgow, of which 
more than half the cases consist of persons with one mark, it is found that the 
mean age in each of the three outbreaks falls steadily as the size of the mark 
