728 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
cord them a high degree of mobility, so that they can be quickly 
dispatched to any part of the body threatened by invaders, and are 
hardly behind the fluids of the blood in this ability to be massed 
or delivered where needed. The phagocytic mechanism of defense 
operates through all the orders of the metazoa; and while it can 
hardly have been developed originally as a protective system against 
parasites, and doubtless represents a mechanism for disposing of 
effete and useless particulate matter in the body by a process of in- 
tracellular digestion, yet it has reached through evolutionary selec- 
tion a high state of perfection and must have exercised no small 
influence in protecting from extinction certain living species. 
There is good reason to believe that in the final disposal of bacteria 
intruded into the body the phagocytes play the terminal role—i. e., 
under favorable conditions they are attracted through chemical 
stimuli furnished by the bacteria to which they respond to englobe 
them, after which the bacteria are often disintegrated. But there is 
_ equally good reason to believe that, with few exceptions, this engulf- 
ing can not take place until the bacteria have been acted on by certain 
plasmatic constituents that prepare the bacteria to be taken into the 
body of the phagocytes. The further the phenomena of bacterial 
destruction in the body are probed the more certain does it become 
that there is no single and uniform process of their disposal. The 
humoral doctrine of bacterial destruction contains much of fact, the 
phagocytic doctrine much of fact, and it is quite certain that the 
practical defensive activities of the body constantly imply the use of 
both mechanisms. 
And when we push the analysis of the manner in which bacteria 
injure the body and enumerate the various bactericidal substances 
which have now been determined as existing in the plasma and in the 
cells, we find that this interaction must be supposed to take place. 
Plasmatic bactericidal action and phagocytic inclusion are coopera- 
tive functions; plasmatic antitoxic action and phagocytic detoxica- 
tion are cooperative functions; plasmatic opsonization and phagocytic 
ingestion are complemental functions; plasmati¢ agglutination and 
phagocytic engulfing are also complemental, although less essential 
functions. And although in intending infections the toxic action of 
the bacteria to be dealt with is less a matter of great consequence, yet 
in principle the disposal of a few bacteria is not different from the 
disposal of many; and in dealing with the poison or toxic elements of 
bacteria, the plasma possesses distinct power of direct neutralization 
as the phagocytes possess distinct ability to transform poisonous into 
nonpoisonous molecules. 
I desire now to refer again to the subject of racial and species 
immunity for which the the humoral factors of bacterial destruction 
afforded an imperfect explanation, in order that I may point out that 
