"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, 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.