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The range is from 1 to —1, the former being complete positive
association and the latter complete negative.
The chance of a head following a tail in an unloaded coin is
recorded as 0. If the penny is loaded, and head follows head,
then the association between the two events will be 1. If the
sequence is always alternate, then the association is —1.
Out of 1,220 couples, as a matter of chance, approximately
in six instances would both be expected to die of Phthisis, the
number actually found was 10, a difference of four.
We find, however, that in mating there is a marked tendencv
for like to mate with like. In the same population as the above
180 couples should have had blue eyes, as an actual fact 240
were found. If any other character had been taken the same
would have been noted. Hence there must be a tendency for the
phthisical, or those likely to become phthisical, to mate.
The conclusion therefore can safely be drawn, that a healthy
man is not likely to be prejudiced or his chance of dying of phthisis
enhanced by marriage to a woman who is already consumptive.
As a corrollary to this it may be stated that infection must occur
before adult life in all cases. Evidence has already been given
which seems to show that the process of infection is complete by
the 14th year.
It is not unreasonable therefore to surmise that the factor
which decides whether immunity can be established with or without
discomfort, and if with discomfort, whether death supervene
or not, is the constitution or diathisis that is inherited from the
parents. It has been conclusively shown that with equal opportunity
of infection a son of a tubercular father is much more
likely to die of that complaint than the sister of a tubercular
brother. The association is of the same order in each case as anv
physical inherited character.