STUUY OF DELAYED OR « LATENT » TUBERCULOUS INFECTION 601 
tuberculosis, the siale of ihe possible victim is of primary 
importance, and tbat Koch’s Bacillus must under normal 
circumstances be considered as a factor of secondary impor¬ 
tance. Tbat there is some element of truth in this naradoxical 
A 
view cannot be entirely denied, but that it is fallacious is quite 
certain, for it is easy to prove thaï Tuberculosis cannot be pro- 
duced without tubercle bacilli, and that ail individuals are not. 
equally exposed and liable to infection. 
A belief in the fixity of the characters and properlies of ail 
the bacilli originally covered by the name of Bacillus tubercu¬ 
lose 5, lias led certain observers to assume the existence of a 
number of species or races of that organism. The quantitative 
pathogenicity of these kinds of bacilli constitutes for these 
observers the chief spécifie différence belween these types. To 
speak only of Tuberculosis as it occurs in three vertebrata, 
there would be a human, a bovine and an avian kind of tubercle 
bacilli with fixed characters. The direct passage of Tubercu¬ 
losis from birds to mammals and vice versa is certainly attended 
with difficulties, and the same may be said to a lesser degree 
of the passage of human bacilli to canine or bovine animais. 
But these difficulties, though usual, are not constant and the 
writer lias observed instances of the direct intercommunicabi- 
lity of tuberculosis between mammals, and belween birds and 
mammals. As to the intercommunicability of tuberculosis 
among mammals, the author’s expérience leaves him no room 
for doubt. In this respect lie agréés with several eminent 
observers and does not think it necessary to do more than state 
his own views before dealing with the spécial object of this 
communication. 
What renders the élucidation of the causes of an infectious 
disease so difficult is that it is necessary to take account of the 
various ways in wliich the predisposing and determining factors 
hâve combined in their action. These-factors may be grouped 
under at least 6 lieads : 
Distribution and habits of the pathogenic organism; 
Conditions inlluencing its number; 
Conditions influencing its virulence (pathogenicily, loxicity); 
Opportunities of access to channels ol infection; 
