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242 BOTANICAL GAZETTE. |Sept., 
plained. Various conjectures have been offered, but no one of 
these to my knowledge has been based upon sufficient direct and 
positive evidence to warrant its acceptance as a well established 
theory of immunity. Since the demonstration of the germ theory 
of contagion, it has been evident that there were, in a general 
way, three possible explanations of acquired immunity, viz: a 
substance might be formed in the body during the course of the 
disease which is unfavorable to the microbes; or a substance es- 
s. 
t is well known that Pasteur has adopted the second or ex- 
haustion theory, and sustains it by his observations on the growth 
of microbes in culture liquids contained in flasks. If we sow 
chicken bouillon, he says, with the microbe of fowl cholera and 
after three or four days filter the liquid in order to remove all 
traces of the microbe, and afterwards sow this parasite again in 
the filtered liquid, it will be found powerless to resume the most 
eeble development. He assumes that there are but two hy poth- 
eses by which this fact can be explained: either the microbe has 
exhausted something from the culture liquid essential to its mul- 
tiplication or it has added some substance which is unfavorable 
to it. To decide between these two possibilities a culture of the 
ma 'y, we find the elements of the problem very materially 
changed. The body is very different from a culture flask to 
which nothing gains entrance and from which nothing is elimi- 
inated. The insusceptible fowl is continually taking into its 
system fresh food which contains principles suited to the growth 
of the microbe in question. If the body is to be compared to a 
culture flask we should expect the immunity to be at the most of 
