He considers that the results of use (that is, increased function) are generally transmitted, because the increase in the organ which is functioning more strongly changes the composition of the blood, by withdrawing from it in a greater degree the specific substances which the organ in question--a muscle, for instance--requires for its activity. All parts of the animal are thereby affected and modified, but especially those smallest vital units or 'fistellæ' (corresponding to biophors) which preside over digestion, and of which there are several sorts. Among them those work most arduously which have to produce the specific substances which serve for the nutrition of the muscles with increased function, because these are needed in larger quantities. This kind of digestive 'fistella' therefore multiplies, while other kinds, whose products are not required and therefore not used up, cease to be so active, diminish in number, and in course of time disappear. In this way the composition of the blood is altered, and with it to a greater or less degree all the characters of the whole organism. Of course the reproductive cells are also under the influence of this change in the composition of the blood, because the different nutritive substances are accumulated within them in an altered proportion corresponding to the changed composition of the blood, the nutritive substances for the muscles with increased function being contained in it in a larger quantity, and thus the greater development of the muscle will repeat itself in the progeny, that is to say, _the acquired character is transmitted_. It is obvious that this is precisely the same line of argument as that used in reference to the origin of the worker and soldier ants. The different kinds of 'digestive fistellæ' correspond to the different food-carrying workers, and the blood to the assumed storehouse from which soldiers and workers select the food suitable for their respective needs, while the sex-cells in the one case, the sexual animals in the other, partake of all kinds exactly in the proportion in which they are stored, and thus the organ which functions most vigorously must be stronger in the offspring.