View report page

106
7. Owing to the existence of these intermediate
viruses, it is found impossible to class all the strains of
mammalian tubercle bacilli which have been investigated
as belonging to one of two stable types.
8. Another significant feature of these intermediate
viruses is that they serve to bridge the apparent gap
between the viruses, on the one hand, which produce acute
tissue destruction in the bovine and rabbit, and are of
relatively scanty growth on culture media, and those viruses
on the other hand, which produce less tissue damage in the
bovine and rabbit and grow more luxuriantly on artificial
media.
A UNITY OF CHARACTERISTICS.
9. Itis found that underlyingall the mammalian viruses
which have been investigated there is an essential unity
of characteristics, the differences observed being differences
of degree, but not of kind. On artificial culture media
they all grow in the same way, though they differ quantitatively
in the amount of growth yielded. In the tissues of
suitable experimental animals they all produce lesions
histologically characteristic of mammalian tuberculosis,
though they differ in the intensity of the tissue changes
which they set up under similar experimental conditions.
10. Comparing the mammalian with the avian viruses,
it is found that there are differences not merely of degree
but of kind.
11. On appropriate culture media the growth of the
avian bacillus can be distinguished without hesitation from
the growth of the mammalian bacillus.
12. Comparing the results of similar experiments
performed under suitable conditions upon suitable animals,