1886. ] BOTANICAL GAZETTE. 243 
but a few days’ duration, since the fresh nutriment should in- 
crease the capacity for growth in the one as well as in the other. 
Immunity from contagious diseases, when once acquired, how- 
ever, does not terminate so soon, and generally persists for years. 
he exhaustion theory is susceptible of being tested by direct 
experiment. Ifa fowl is insusceptible to cholera because it lacks 
some element essential to the growth of the microbe, then bouillon 
made by infusing the muscles of this fowl in distilled water 
should also lack this same element and would therefore be equally 
incapable of nourishing the germ. In February, 1881, the writer 
was investigating the subject of fowl cholera, and made this ex- 
periment; and he found that the proliferation of the microbe was 
Just as vigorous in bouillon made from insusceptible fowls as in 
that made from susceptible ones (Rep. U. S. Dep. Ag., 1881 
and 1882, p. 292). 
_ Both the antidote and the exhaustion theory, consequently, 
fail when tested by direct experiment ; indeed when we consider 
that there must be a different chemical substance exhausted from 
the body for each contagious disease against which immunity is 
acquired in the one case, or a different product for each disease 
added in the other case, the theories become at once improbable. 
f we direct our attention now to the third or vital resist- 
ance theory, such discrepancies in regard to well established facts 
will not be found. Immunity is probably never absolute, but 
simply relative. Chauveau found that the Algerian sheep, sup- 
posed to be insusceptible to charbon, would succumb to that dis- 
ease if a sufficiently large dose of virus was administered, and 
the writer found that fowls insusceptible to ordinary doses of 
disease if the dose was suffi- 
increasing the dose the resist~ 
overcome, the microbes multi- 
If the germs failed to multiply 
ance of the tissues is in some way 
ply and the disease is produced. 
