31 
have shown that it is easier to infect younger animals with certain 
species of parasites than it is to infect older animals. In these cases 
an actual physiological predisposition is assumed, and it is apparently 
not illogical to assume a greater physiological difference between a 
child of 10 and an adult of 50 than between a male of 50 and a female 
of 50. Accordingly, in the case of children, it would appear that we 
have to deal not only with a question of the relative intimacy existing 
between the person and the infectious material, but also (at least in 
case of some species of parasites) with a physiological predisposition, 
due to the more susceptible condition of the tissues in younger hosts. 
That the total columns should therefore show the highest percen- 
tage of infection under 15 years of age in both males and females is a 
fact due apparently both to physiology and to environment. But it 
is perfectly conceivable that environment (as in the case of miners, 
or of the soldiers from the Philippines, or in rural localities presenting 
special conditions) may play a much more important role than that 
played by physiological predisposition. 
As the individual grows older this element of physiological predis- 
position would become progressively less and that of environment 
greater in importance, so that in persons over 15 years of age the 
prevalence of intestinal worms among persons of different ages, as 
also among the two sexes, would appear to be governed more by the 
degree of intimacy between persons of a given age and the source of 
infection and less by physiological conditions pertaining to that age. 
In other words, the difference in physiological predisposition between 
persons under 15 years -of age and any older age group would be 
greater than this difference between any two older groups; hence it 
would play a smaller part as a factor in determining the prevalence of 
intestinal worms among the older groups. 
It would appear that the intimacy between persons of different 
ages (above 15 years) and the concentrated infection found in the 
immediate vicinity of the dwelling in rural districts would not be the 
same in the two sexes. Up to about 15 years of age both males and 
females would be in close contact with this highly infected soil about 
the house, the girls naturally being somewhat more closely confined 
to this area than the boys. After 15 years of age, however, which is 
probably about the average age at which school life is ended in rural 
districts, 'the females would, as they became more and more confined 
to domestic duties with advancing years, likewise become more and 
more confined to the area of most concentrated infection about the 
dwelling; this would be true, at least until they reached such ad- 
vanced age as would preclude their doing any work outdoors and 
restrict their work pretty closely to the house itself. The males, 
however, upon the ending of school days, would go to work in the 
fields and come less in contact with the immediate environment of the 
