1910-11.] The Mono-Molecular Reaction and Life-Processes. 633 
rate increases in a geometrical progression, it may be expected that some 
substance essential to vitality disappears in the organisms, according to the 
assumed law. In this way protection against fevers becomes more 
difficult with each year of age. So that the loss of immunity is due to the 
inability of the organisms to furnish protective substances. It is therefore 
distinct from case 1. With case 4 something essentially similar takes 
Diagram II. 
place ; the attacking organism in this case loses something necessary 
to the power of attack, and this lost, the epidemic terminates for the 
moment till by some means, possibly sexual, the vitality of the organ- 
ism is regained. With some epidemic diseases this seems roughly cyclical. 
The fifth case is specially interesting as showing that acquired immunity 
passes in the same way, and that here also the substances in the body 
which control the reaction to disease behave essentially as if the loss 
of power were due to rearrangement of the various atoms of individual 
molecules. 
