In the whole history of humanity there has never been a time when this question of heredity has been discussed so minutely and in so many different ways as it has been in the present century. Although this theory was known in the East by the ancient Vedanta philosophers, by the Buddhists of the pre-Christian era and by the Greek philosophers in the West, still it has received a new impetus and has grown with new strength since the introduction of the Darwinian theory of the evolution of species. Along with the latest discoveries in physiology, biology, embryology and other branches of modern science, the popular simple meaning of heredity--that the offspring not only resemble their parents among animals as well as among men, but inherit all the individual peculiarities, life and character of their parents--has taken the shape of the most complicated and difficult problem which it is almost impossible to solve. Our minds are no longer satisfied with Haeckel's definition that heredity is simply an overgrowth of the individual, a simple continuity of growth; but we want to know the particular method by which hereditary transmission takes place. We ask, how can a single cell reproduce the whole body of the offspring, its mind, character and all the peculiarities of an organism? Out of the myriads of cells of which a body is composed, what kind of cell is that which possesses the power of reproducing the peculiarities, both mental and physical, which are to be found in the form of the new-born babe? This is the most puzzling of all the problems which the scientific mind has ever encountered. The fundamental question connected with the theory of heredity is: How can a single cell of the body contain within itself all the hereditary tendencies of the hypothesis of the continuity of the germ-plasm gives an identical starting-point to each successive generation, and thus explains how it is that an identical product arises from all of them. In other words, the hypothesis explains heredity as part of the underlying problems of assimilation and of the causes which act directly during ontogeny. (Vol. I, p. 170.) According to Weismann, all the peculiarities which we find in an organism are not inherited by the organism from that of the parents, but he says: "Nothing can arise in an organism unless the predisposition to it is pre-existent, for every acquired character is simply the reaction of the organism upon a certain stimulus." (Vol. I, p. 172.) Therefore the germ-cells do not inherit all the peculiarities of the parents, but possess the predisposition or a potentiality of the tendencies which gradually develop into individual characters.