SUSCEPTIBILITY AND IMMUNITY, 253 
decomposition will soon be perceived. But if we inject a like quantity of 
the culture fluid with its contained bacteria into the circulation of a living 
animal, not only does no increase and no putrefactive change occur, but the 
bacteria introduced quickly disappear, and at the end of an hour or two the 
most careful microscopical examination will not reveal the presence of a 
single bacterium. This difference we ascribe to the vital properties of the 
fluid as contained in the vessels of a living animal; and it seems probable 
that the little masses of protoplasm known as white blood corpuscles are the 
essential histological elements of the blood, so far as any manifestation of 
vitality is concerned. The writer has elsewhere (1881) suggested that the 
disappearance of the bacteria from the circulation, in the experiment 
referred to, may be effected by the white corpuscles, which, it is well known, 
pick up, after the manner of amcebe, any particles, organic or inorganic, 
which come in their way. And it requires no great stretch of credulity to 
believe that they may, like an amoeba, digest and assimilate the protoplasm 
of the captured bacterium, thus putting an end to the possibility of its do- 
ing any harm. 
‘In the case of a pathogenic organism we may imagine that, when cap- 
tured in this way, it may share a like fate if the captor is not paralyzed by 
some potent poison evolved by it, or overwhelmed by its superior vigor and 
rapid multiplication. In the latter event the active career of our conserva- 
tive white corpuscle would be quickly terminated and its protoplasm would 
serve as food for the enemy. It is evident that in a contest of this kind the 
balance of power would depend upon circumstances relating to the inherited 
vital characteristics of the invading parasite and of the invaded leucocyte.” 
In the same chapter the writer quotes from his paper on acquired 
immunity, published in 1881, as follows : 
‘‘ The difficulties into which this hypothésis [the exhaustion theory of Pas- 
teur] leads us certainly justify us in looking further for an explanation of the 
phenomena in question. This explanation is, I believe, to be found in the 
peculiar properties of the protoplasm, which is the essential framework of 
every living organism. The properties referred to are the tolerance which 
living protoplasm may acquire to certain agents which, in the first instance, 
have an injurious or even fatal influence upon its vital activity ; and the 
property which it possesses of transmitting its peculiar qualities, inherent or 
acquired, through numerous generations, to its offshoots or progeny. 
‘*Protoplasm is the essential living portion of the cellular elements of ani- 
mal and vegetable tissues; but as our microscopical analysis of the tissues has 
not gone beyond the cells of which they are composed, and is not likely to 
reveal to us the complicated molecular structure of the protoplasm, upon 
which, possibly, the properties under consideration depend, it will be best, 
for the present, to limit ourselves to a consideration of the living cells of the 
body. ‘These cells are the direct descendants of the pre-existent cells, and 
may all be traced back to the sperm cell and the germ cell of the parents. 
Now, the view which I am endeavoring to elucidate is that, during a non- 
fatal attack of one of the specific diseases, the cellular elements implicated, 
which do not succumb to the destructive influence of the poison, acquire a 
tolerance to this poison which is transmissible to their progeny, and which 
is the reason of the exemption which the individual enjoys from future 
attacks of the same disease. 
‘“The known facts in regard to the hereditary transmission by cells of ac- 
quired properties make it easy to believe in the transmission of such a 
tolerance as we imagine to be acquired during the attack; and if it is shown 
by analogy that there is nothing improbable in the hypothesis that such a 
tolerance is acquired, we shall have a rational explanation, not of heredity 
and of the mysterious properties of protoplasm, but of the particular result 
under consideration. The transmission of acquired properties is shown in 
the budding and grafting of choice fruits and flowers, produced by cultiva- 
