When we take the idea of regeneration, or the recuperation of parts that have been rendered useless, in the widest sense, we find it to be a universal vital function of the greatest importance. The whole metabolism of the living organism consists in the assimilation of plasm, or the replacing of the plasma-particles which are constantly used up by dissimilation (_cf._ chapter x.). Verworn has given the name of _biogens_ to the hypothetical molecules of living matter--which I regard with Hering as endowed with memory, and (1875) have called plastidules. He says: "The biogens are the real vehicles of life. In their constant decay and reconstruction consists the process of life, which expresses itself in the great variety of vital phenomena." The relation of assimilation (the building-up of the biogens) to dissimilation (the decay of the biogens) may be expressed by a fraction to which the name _biotonus_ is given A/D. It is of radical importance in the various phenomena of life. The variations in the size of this fraction are the cause of all change in the life-expression of every organism. When the biotone increases, and the metabolism quotient becomes more than one, we have growth; when, on the other hand, it falls below one, and the biotone decreases, we have atrophy, and finally death. New biogens are constructed in _regeneration_. In _generation_ or reproduction groups of biogens (as germ-plasm) are released from the parent in consequence of redundant growth, and form the foundation of new individuals.