In investigating this question, we need first, if possible, to locate the bearers of hereditary qualities within the physical substance that connects one generation with the next; then, to study their behavior during the transmission of life in order to account for the inheritance of both maternal and paternal qualities; and, lastly, to determine whether or not transiently acquired characteristics are inherited. Hereditary Qualities in the Germinal Elements.--When we take into consideration the fact established for all animals and plants (setting aside cases of budding and the division of unicellular organisms), that the only substance that passes from one generation to another is the egg and the sperm in animals, and their representatives in plants, we see that the first question is narrowed to these bodies. If all hereditary qualities are carried in the egg and the sperm--as it seems they must be--then it follows that these germinal elements, although microscopic in size, have a very complex organization. The discovery of this organization must depend upon microscopic examination. Knowledge regarding the physical basis of heredity has been greatly advanced by critical studies of cells under the microscope and by the application of experimental methods, while other phases of the problems of inheritance have been elucidated by the analysis of statistics regarding hereditary transmissions. The whole question, however, is so recent that a clear formulation of the direction of the main currents of progress will be more helpful than any attempt to estimate critically the underlying principles. Early Theories.--There were speculations regarding the nature of inheritance in ancient and mediæval times. To mention any of them prior to the eighteenth century would serve no useful purpose, since they were vague and did not form the foundation upon which the modern theories were built. The controversies over pre-formation and epigenesis (see Chapter X) of the eighteenth century embodied some ideas that have been revived. The recent conclusion that there is in the germinal elements an inherited organization of great complexity which conditions inheritance seems, at first, to be a return to the doctrine of pre-formation, but closer examination shows that there is merely a general resemblance between the ideas expressed by Haller, Bonnet, and philosophers of their time and those current at the present time. Inherited organization, as now understood, is founded on the idea of germinal continuity and is vastly different from the old theory of pre-formation. The meaning of epigenesis, as expressed by Wolff, has also been modified to include the conception of pre-localization of hereditary qualities within particular parts of the egg. It has come now to mean that development is a process of differentiation of certain qualities already laid down in the germinal elements.