length an unfit envelope. Somatic death then follows as a natural consummation, but the germ-cells are, in a sense, immortal in that they remain capable of indefinite growth by division. In the sexual reproduction of the higher organism a part of the germ-plasm becomes detached, undergoes growth, and develops into an organism exhibiting the parental organisation. But in the development of the offspring, part of the germ-plasm received from the parent persists unchanged, is transmitted to another generation, and so on without apparent limit. _Something is transmitted from parent to offspring._ This something we regard as a cell exhibiting a definite chemical and physical structure; but while the germ-cell differs in certain respects from an ordinary somatic cell, these structural and chemical differences are insignificant when they are compared with the differences in the potentialities of the cells. The somatic cells are, in general, capable of reproducing only the general character of the tissues of which they form part. Some of them, the cells of the grey matter of the central nervous system, for instance, appear to be incapable of division and growth. But again the facts of regeneration appear to point to the possession by the somatic cells of more than this restricted power of reproducing the tissues of which they form part: to this extent the regeneration experiments tend to remove the essential distinction between the somatic and germinal cells. Neglecting these results in the meantime, we see that the germ-cells contain within themselves the potentiality of reproducing the entire organism in all its specificity. That which is transmitted from the parent to the offspring is the parental organisation in all its specificity; and to say that this organisation is a material thing is, of course, to state a hypothesis, not a fact of observation. This transmission of a specific form and mode of behaviour from generation to generation is what a hypothesis of heredity attempts to explain--that is, to describe in the simplest possible terms, making use of the concepts of physical science. “Twelve years ago,” says Jacques Loeb, “the field of heredity was the stamping ground for the rhetorician and metaphysician; it is to-day perhaps the most exact and rationalistic part of biology, where facts cannot only be predicted qualitatively, but also quantitatively.” Let the reader examine for himself the meagre array of facts on which this apotheosis of mechanistic biology is based.