IMMUNITY IN CALVES AGAINST RINDERPEST               145

      The relation of age to lack or loss of immunity is particularly well marked in the
European crossbred animals, although not so evident in the calves of indigenous
breeds, probably on account of the smaller numbers involved in the age groups in
which breakdowns occurred.

      Further there is with increasing age at the time of inoculation a decrease in the
severity of the reaction to the test dose. The only animal in the whole series which
died as a direct result of rinderpest was inoculated when 14 days old and tested 31
months later, which would indicate quite a wide range in the degree of immunity
conferred by the original inoculation. Owing to inequality of numbers in the various
age groups as compared with the period of test it is not possible to decide whether
the immunity conferred originally wanes proportionately to the interval allowed
before test or whether the evidence of incomplete immunity at the time of test
indicates that only this degree of immunity was conferred at the time of the
first inoculation.

      We are inclined to believe that the immunity conferred by the first inocula-
tion does not remain constant but decreases more or less rapidly, depending on the
individual, until the stage of complete susceptibility is once more reached.