Although Professor Weismann denies that acquired characters, that is, individual peculiarities arising as the result of personal experience, are transmitted, he admits that congenital characters, that is, peculiarities with which an individual is born, are transmitted to offspring. As congenital characters must, originally, have been individual, it is not easy at first sight to perceive Weismann's real meaning. It is necessary, therefore, to enter more particularly into a consideration of his theory, which he regards as in general accord with Darwin's theory of pangenesis. Darwin supposes that all the cells of the body continually give off great numbers of _gemmules_, which are conveyed by the blood and deposited in the germ-cells of the organism. These cells are thus endowed with the power of developing a new organism of the same kind, each gemmule reproducing the cell from which it was derived. These ultimate vital units are called by Weismann _biophors_, but he supposes them not to be the ultimate "bearers of vitality." They are said to be arranged in groups to which the term _determinants_ is applied, and these groups are combined so as to form ancestral _ids_ or germ-plasms. Each determinant, which is made up of perfectly definite numbers and combinations of biophors, is the primary constituent of a particular cell, or of a group of cells, such as a blood corpuscle. The determinants thus "control the cell by breaking up into biophors, which migrate into the cell body through the nuclear membrane, multiply there, arrange themselves according to the forces within them, and determine the histological structure of the cell," impressing upon it its inherited specific character. The structure of the cell, and of every subsequent stage, exists therefore potentially in the inherited structure of the id, and the determination of its character "depends on the biophors which the corresponding determinant contains, and which it transmits to the cell."