IMMUNITY IN TUBERCULOSIS—FLEXNER. 629 
will recall the long list of cultures which is given in the paper on 
tuberculosis’ published in 1884. In regard to this list Koch Says: 
“ It may cause some surprise that so relatively large a number of cul- 
tures was set on foot when a few would have sufficed for observing 
the behavior of bacilli in cultures. It seemed to me, however. not 
improbable that though bacilli from varying forms of tuberculosis— 
perlsucht, lupus, phthisis, etc., presented no differences microscopic- 
ally, yet, that in cultures differences might become apparent between 
bacilli from different sources. But although I devoted the greatest 
attention to this point, I could find nothing of the kind. In all the 
cultures, whether taken from miliary tubercles, lupus, or perlsucht, 
the tubercle bacilli behaved exactly the same.” 
Our knowledge of the nature of the tubercle bacillus has been in- 
creased until at this time several distinct kinds are recognized. These 
may conveniently be classified according to their chief sources into 
human, bovine, and avian tubercle bacilli, and into so-called tubercle 
bacilli of cold-blooded animals. This last group of bacilli, which 
will detain us only a short time, differs greatly from the other vari- 
eties, as can readily be seen when the fact is recalled that the high 
temperatures—temperatures approaching blood heat—which are: re- 
quired for the growth of the mammalian and avian bacilli, quite pre- 
clude their multiplication under conditions of ordinary external 
nature. Hence they are not adapted to a life outside the living body 
except as cultivated artificially at this relatively high temperature. 
In man’s conflict with tuberculosis this fact is of the greatest service, 
since by reason of it he is enabled to disregard the danger of any 
increase in tubercle bacilli outside the animal body. The relatively 
low temperatures at which the tubercle bacilli of cold-blooded animals 
develop adapt them, indeed, to an independent existence; but, as they 
are wholly devoid of power to cause disease in warm-blooded animals 
and as they would appear to have a restricted dissemination even 
among cold-blooded species, they are of comparatively small impor- 
tance. 
Of far greater consequence is the question whether the disparity 
which exists between the several kinds of tubercle bacilli derived from 
warm-blooded animals is a wide one. This question, which at first 
sight may appear to be chiefly of academic interest, has, in reality, 
far-reaching practical significance. The close relationship which man 
bears to domestic animals makes every fact of animal disease of high 
value to him. And in the case of no animal disease are facts of greater 
moment than in tuberculosis. Not only is the human race, by reason 
of its dependence upon the animal kingdom for food, work, etc., ex- 
posed to the diseases of animals which are transmissible to man, but 
domestic animals are also exposed to diseases of human beings. This 
correlative susceptibility may, therefore, cooperate to produce a 
