254 SUSCEPTIBILITY AND IMMUNITY. 
tion, upon the wild stock from which they originated. The acquired proper- 
tics are transmitted indefinitely; and the same sap which on one twig nour- 
ishes a sour crab apple, on another one of the same branch is elaborated into 
a delicious pippin. 
‘The tolerance to narcoties—opium and tobacco—and to corrosive poisons 
—arsenic—which results from a gradual increase of dose, may be cited as an 
example of acquired tolerance by living protoplasm to poisons which at the 
outset would have been fatal in much smaller doses. 
_ ‘The immunity which an individual enjoys from any particular disease 
must be looked upon as a power of resistance possessed by the cellular ele- 
ments of those tissues of his body which would yield to the poison in the 
case of an unprotected person.” . 
This theory of immunity, advanced by the author in 1881, has 
received considerable support from investigations made since that 
date, and especially from the experimental demonstration by Sal-. 
mon, Roux, and others that, as suggested in the paper from which I 
have quoted, immunity may result from the introduction into the 
body of a susceptible animal of the soluble products of bacterial 
growth—filtered cultures. 
The theory of vital resistance to the toxic products evolved by 
pathogenic bacteria is also supported by numerous experiments 
which show that natural or acquired immunity may be overcome 
when these toxic products are introduced in excess, or when the vital 
resisting power of the animal has been reduced by various’ agencies. 
More direct experimental evidence in favor of the view under con- 
sideration is that obtained by Beumer in his experiments with steril- 
ized cultures of the typhoid bacillus. He found that after the re- 
peated injection of non-lethal doses mice were able to resist an 
amount of this toxine which was fatal to animals of the same spe- 
cies not so treated. But, on the other hand, Gamaléia found, in his 
experiments upon guinea-pigs which had been made immune against 
the pathogenic action of a spirillum, called by him Vibrio Metschni- 
kovi, that these animals have no increased tolerance for the toxic 
products of this microédrganism. Although immune against infec- 
tion by the living microbe, they were killed by the same quantity of 
a sterilized culture as was fatal to guinea-pigs which had not been 
rendered immune. 
Charrin has obtained similar results in experiments with filtered 
cultures of Bacillus pyocyaneus. Rabbits which had an artificial im- 
munity against the pathogenic action of thé bacillus were killed by 
doses of a sterilized culture such as were fatal to other rabbits of the 
same size not immune. In subsequent experiments by Charrin and 
Gameléia “vaccinated” rabbits were found to be even more suscepti- 
ble to the toxic action of filtered cultures than were those not vacci- 
nated. Metschnikoff (1891) has followed up this line of experiment, 
and has shown that when considerable amounts of filtered cultures 
of Bacillus pyocyaneus are injected subcutaneously in rabbits a cer- 
