124 The American Naturalist. [February, 
be left unmolested to pursue its ravages on helpless humanity, 
But in reality it was being followed up by tireless and relentless 
foes. 
On October 19th, 1889, was published in the Medical News, 
of Philadelphia, by Dr. Samuel G. Dixon, at that time Professor 
of Hygiene to the University of Pennsylvgnia, a monograph 
announcing his discovery of the hitherto-unknown forms of the 
tubercle bacillus. 
In the previous summer, whilst investigating different methods 
of technique and manipulation abroad, Dr. Dixon was led to 
believe that the bacillus could be cultivated so as to show lower 
forms of virulent life; and following this idea up by a series of 
experiments, he was in a short time able to produce the hitherto- 
unnoticed forms of the bacilli, some club-shaped, others curved, 
and others again branched. 
From the growths thus obtained he proceeded to make a series 
of tube inoculations, from which he grew bacilli corresponding ™ 
every respect to the ordinary rod-shaped tubercle bacillus. 
Having obtained these results, he propounded two hypotheses: 
Ist, That by a thorough filtering out of bacilli from tuberculous 
material a filtrate might be obtained and attenuated, so that ws 
systematic inoculations a change might be produced in living 
animal tissues that would enable them to resist virulent tubercle 
bacilli. 2d, To bring about a chemical change or physical change 
in living tissues that would resist tubercular phthisis, it is possible 
that inoculation with the bacillus would have to be made; ee 
before this could be done, the power of the virulent bacilli would , 
have to be diminished, otherwise the result would be most disas- 
trous. He added further that he had reduced the tubercle bacilli 
to a condition that, when inoculated into the animal economy 
caused a resistance to the disease. ae 
To use a military metaphor, this was the first note proclait 1 
that an active campaign had been opened on the tubercle pee 
and specifying in terms as definite as possible the means se T 
the war was carried into the enemy’s country. = a 
The announcement of this discovery was widely circulated 
commented upon, and reprints of the article were fo sae 
