vice versa, _will die in
surroundings where its sister species might live and thrive_.

One of the first differences found to exist among these three types of
bacteria was the extraordinary variation in their power of attacking
different animals. For instance, while the guinea-pig and the rabbit
could be readily inoculated with _human_ bacilli, they could only be
infected with difficulty by cultures of the _bovine_ bacillus; while the
only animal that could be inoculated at all with the _avian_ or bird
bacillus was the rabbit, and he only occasionally. In fact,
bacteriologists soon came to the consoling conclusion that the _avian_
bacillus might be practically disregarded as a source of danger to human
beings, so widely different were the conditions in their moist and
moderately warm tissues to those of the dry and superheated tissues of
the bird to which it had adjusted itself for so many generations.

And next came the bold pronunciamento of no less an authority than Koch
himself, that the bovine bacillus also was so feebly infective to human
beings that it might be practically disregarded as a source of danger.
This promptly split the bacteriologists of the world into two opposing
camps, and started a warfare which is still being waged with great
vigor. As the question is still under hot dispute by even the highest
authorities, it is, of course, impossible to pronounce any definite
conclusions. But the net result to date appears to be that while Koch
made a serious error of judgment in declaring that meat and milk as a
source of danger to human beings of tuberculosis might be disregarded,
yet, for practical purposes, his position is, in the main, correct: the
actual danger from the bovine bacillus to human beings is relatively
small.

There was nothing whatever improbable, in the first place, in the
correctness of Koch's position.

It is one of the few consoling facts, well known to all students of
comparative pathology or the diseases of the different species of
animals, how peculiarly specialized they are in the choice of their
diseases, or, perhaps, to put it more accurately, how particular and
restricted disease-germs are in their choice of a host. For instance,
out of twenty-eight actually infectious diseases which are most common
among the domestic animals and man, other than tuberculosis, only
one--_rabies_--is readily communicable to more than three species; only
three--_anthrax_, _tetanus_, and _foot-an