Exposition of the Doctrine of Germinal Continuity.--From parent to offspring there passes some hereditary substance; although small in amount, it is the only living thread that connects one generation with another. It thus appears that there enters into the building of the body of a new organism some of the actual substance of both parents, and that this transmitted substance must be the bearer of hereditary qualities. Does it also contain some characteristics inherited from grandparents and previous generations? If so, how far back in the history of the race does unbroken continuity extend? Briefly stated, genetic continuity means that the ovum and its fertilizing agent are derived by continuous cell-lineage from the fertilized ovum of previous generations, extending back to the beginning of life. The first clear exposition of this theory occurs in the classical work of Virchow on _Cellular Pathology_, published in 1858. Virchow (1821-1902), the distinguished professor of the University of Berlin, has already been spoken of in connection with the development of histology. He took the step of overthrowing the theory of free cell-formation, and replacing it by the doctrine of cell-succession. According to the theory of Schleiden and Schwann, cells arose from a blastema by a condensation of matter around a nucleus, and the medical men prior to 1858 believed in free cell-formation within a matrix of secreted or excreted substance. This doctrine was held with tenacity especially for pathological growths. Virchow demonstrated, however, that there is a continuity of living substance in all growths--that cells, both in health and in disease, arise only by the growth and division of previously existing living cells; and to express this truth he coined the formula "_omnis cellula e cellula_." Manifestly it was necessary to establish this law of cell-succession before any idea of germinal continuity could prevail. Virchow's work in this connection is of undying value. When applied to inheritance the idea of the continuity of living substance leads to making a distinction between germ-cells and body-cells. This had been done before the observations of Virchow made their separation of great theoretical value. Richard Owen, in 1849, pointed out certain differences between the body-cells and the germinal elements, but he did not follow up the distinction which he made. Haeckel's _General Morphology_, published in 1866, forecasts the idea also, and in 1878 Jaeger made use of the phrase "continuity of the germ protoplasm." Other suggestions and modifications led to the clear expression by Nussbaum, about 1875, that the germinal substance was continued by unbroken generations from the past, and is the particular substance in which all hereditary qualities are included. But the conception finds its fullest expression in the work of Weismann.