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composition and constitution of the blood; the white corpuscles may be destroyed, or their movements enfeebled, and, by any such changes, the blood, which previously could resist the attacks of the various parasitic microorganisms, is so modified as to render it a suitable soil in which these organisms may develope and thrive. This view is confirmed by the experiments of Rossbach and Rosenberger, who found that when papain or sterilised septic blood was injected into the vessels, micrococci developed in the blood with extraordinary rapidity. The blood, that is to say, was so modified in character that the micrococci which could gain no entrance previous to the injection of papain, afterwards found a congenial soil for their development.
We may take another illustration from the process of fermentation. By fermentation sugar can be converted into carbonic acid and alcohol; but a pure saccharine solution does not ferment on the addition of a small quantity of fungi or bacteria in a pure state. Some nitrogenous material must be added as well, to act as food to the organisms, and then the transformation into carbonic acid and alcohol commences.
That it is only in particular conditions of a tissue, or at special times in its growth, that it furnishes suitable soil for the growth of the bacilli is shown by the following illustration from the vegetable world, furnished by de Bary:—
"The common garden-cress Lepidium sativum, is often attacked by a parasitic fungus of comparatively large size, Cystopus candidus. In consequence of this it shows considerable degeneration, swellings, curvatures of the stem, and often also of the fruits, and on these parts and on the leaves white spots and pustules subsequently turning to dust, which are formed by the sporogenous organs of the Cystopus, and give the entire pheno-
