Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/146

Experiments which have been carried on with different so-called toxins have shown, without any doubt, that splitting agents are present in them. These experiments, however, do not make it absolutely certain that the micro-organisms secrete ferments, because it is difficult to decide whether the so-called toxins of the trade represent uniform products, and particularly whether they always contain secretions only. As a preliminary condition, then, for the possibility of the existence of micro-organisms amidst a particular complex of cells foreign to them, we require the presence of ferments which enable them to build up the food they require from substances that are in harmony with the cells and the blood of their host. In this case the relations between the configuration of the ferments and that of the substrates are no doubt expressed in their most distinctive form. How often may not a micro-organism penetrate into an organism and die out, only because it is unable to feed on the nutritive medium supplied, while in other cases it is able to settle down because the substrates presented to it can be rendered accessible by its ferments. If all the substances are used up, and none of the same kind are supplied anew by the host at the proper place, then the conditions of its existence are withdrawn from the micro-organism, and it must either perish or find a new "pasture." It may also happen in many cases that