It is a favorite theory with some modern writers that the physical change from youth to age is accompanied in the body, and in a sense caused by, the deterioration in the quality of the cells of the body, and they call this change a breaking-down process by which the finer and more highly differentiated cells, such, for example, as the nerve-cells, and others which have high and complicated duties to perform, are displaced by cells of an inferior type, which they name conjunctive cells, much as the common sparrow drives away the songbirds from the home garden and, usurping the place of the songbird, substitutes a wretched twitter for the golden notes of the warblers which once delighted our ears. The common cells, also, on usurping the place of the nobler cells, are unable to perform the difficult duties of the latter, and the result upon human organism is disorder, decay, disease, etc., contributing to, if not causing, the condition of old age. This is an ingenious but not convincing theory. Our knowledge of histological processes is too incomplete at this stage to permit its acceptance as fact. It assumes too much to be known which is quite unknown. Moreover, it refutes itself upon examination in this particular, and in several others, that if it were true that these inferior cells are on the lookout to invade instantly any part of the human organism in which there was a breaking down of nerve-tissue, for example, then it would be impossible to build new nerve-tissue to take the place of that which was destroyed, because its place, according to this theory, has been already taken by an intruder who cannot be dislodged. But new nerve-cells are constantly being rebuilt, and constantly being put to use in the organism. If this theory were true, then a brain in middle age would be unable to function because of the impossibility of renewing its cells.