The studies were carried further into the diseases of the higher animals, and it was found the leucocytes in these played the same part as did the cells in the body cavity of the daphnea.

The introduction of bacteria into certain animals was followed by their destruction within cells and no disease resulted; if this did not take place, the bacteria multiplied and produced disease.
Support also was given the theory by the demonstration at about the same time that in most of the infectious diseases the leucocytes of the blood became increased in number,--that in pneumonia, for instance, instead of the usual number of eight thousand in a cubic millimeter of blood, there were often thirty thousand or even fifty thousand.

At about the same time also chemotaxis, or the action of chemical substances in attracting or repelling organisms, excited attention, and all these facts together became woven into the theory.
It was soon seen, however, that this theory, based as it was on observation and supported by the facts observed, was not, at least in its first crude form, capable of general application.

Many animals have natural immunity to certain diseases; they do not have the disease under natural conditions, nor do they acquire the disease when the organisms causing it are artificially introduced into their tissues by inoculation.

Such natural immunity seemed to be unconnected with defence by phagocytosis, for the leucocytes of the animal might or might not have phagocytic reaction to the particular organisms to which the animal was immune.