ation to the surroundings in which they find
themselves, namely, the tissues and body-fluids of some particular host
whom they attack, bring certain limitations with them. Just in so far as
they have adjusted themselves to live in and overcome the opposition of
the body-tissues of a certain species of animals, _just to that degree
they have incapacitated themselves to live in the tissues of any other
species_.

Some of the most interesting and far-reachingly important work that has
been done in the bacteriology of tuberculosis of late years has
concerned itself with the changes that have taken place in different
varieties and strains of tubercle bacilli as the result of adjusting
themselves to particular environments. The subject is so enormous that
only the crudest outlines can be given here, and so new that it is
impossible to announce any positive conclusions. But these appear to be
the dominant tendencies of thought in the field so far.

Though nearly all domestic animals and birds, and a majority of wild
animals under captivity, are subject to the attack of tuberculosis,
practically all the infections hitherto studied are caused by one of
three great varieties or species of the tubercle bacillus: the _human_,
infesting our own species; the _bovine_, attacking cattle; and the
_avian_, inhabiting the tissues of birds, especially the domestic fowl.
These three varieties or species so closely resemble one another that
they were at one time regarded as identical, and we can well remember
the wave of dismay which swept over the medical world when Robert Koch
announced that the "_perlsucht_" of cattle was a genuine and
unquestioned tuberculosis due to an unmistakable tubercle bacillus. But
as these varieties were thoroughly and carefully studied, it was soon
found that they presented definite marks of differentiation, until now
they are universally admitted to be distinct varieties, each with its
own life peculiarities, and, according to some authorities, even
distinct species.

"But," we fancy we hear some one inquire impatiently, "what do those
academic, technical distinctions matter to us? Whether the avian
tuberculosis germ is a variety or a true species may be left to the
taxonomists, but it is of no earthly importance to us."

On the contrary, it is of the greatest importance. For the distinctive
feature about a particular species of parasite is that it will live and
flourish where another species will die, and,