244 BOTANICAL GAZETTE. |Sept., 
when a small number were introduced, because there was some- 
thing lacking in the constituents of the body which is essential 
to their growth, it is difficult to understand how this unfavorable 
condition can be overcome by increasing the dose of virus; or 
if the failure to multiply was due to the existence of some sub- 
stance which acts as a poison to the microbe, it is equally diffi- 
cult to conceive how a large dose of virus would insure pro- 
liferation when a small one fails. 
ith these various facts in mind, we are prepared to understand 
how immunity results from one attack of a contagious disease. 
erance is gained the tissues resume their vital functions as before, 
the liquids of the body become unfavorable to the existence of 
sufficiently increasing the dose of tobacco, so the immunity of 
most individuals may be overcome by administering a very large 
[The discussion of observations and theories made by Metschnikopf, Chau- 
veau, Ziilger, Riemschneider, Hiller, Pasteur, and the author, which bear upon 
the elucidation of the subject, but do not affect its general statement, have been 
omitted for lack of Space.— Eps. ] 
the preventive treatment of contagious diseases. Investigations 
of this question have not been as numerous or thorough as is de- 
sirable. Pasteur found that his fowls which had been treated 
