Darwin's Theory of Pangenesis.--In attempting to account for heredity, Darwin saw clearly the necessity of providing some means of getting all hereditary qualities combined within the egg and the sperm. Accordingly he originated his provisional theory of pangenesis. Keeping in mind the fact that all organisms begin their lives in the condition of single cells, the idea of inheritance through these microscopic particles becomes difficult to understand. How is it possible to conceive of all the hereditary qualities being contained within the microscopic germ of the future being? Darwin supposed that very minute particles, which he called gemmules, were set free from all the cells in the body, those of the muscular system, of the nervous system, of the bony tissues, and of all other tissues contributing their part. These liberated gemmules were supposed to be carried by the circulation and ultimately to be aggregated within the germinal elements (ovum and sperm). Thus the germinal elements would be a composite of substances derived from all organs and all tissues. With this conception of the blending of the parental qualities within the germinal elements we can conceive how inheritance would be possible and how there might be included in the egg and the sperm a representative in material substance of all the qualities of the parents. Since development begins in a fertilized ovum, this complex would contain minute particles derived from every part of the bodies of both parents, which by growth would give rise to new tissues, all of them containing representatives of the tissues of the parent form. Theory of Pangenesis Replaced by that of Germinal Continuity.--This theory of Darwin served as the basis for other theories founded upon the conception of the existence of pangens; and although the modifications of Spencer, Brooks, and others were important, it is not necessary to indicate them in detail in order to understand what is to follow. The various theories founded upon the idea of pangens were destined to be replaced by others founded on the conception of germinal continuity--the central idea in nineteenth-century biology. The four chief steps which have led to the advancement of the knowledge of heredity, as suggested by Thomson, are as follows: "(a) The exposition of the doctrine of germinal continuity, (b) More precise investigation of the material basis of inheritance, (c) Suspicions regarding the inheritance of acquired characteristics, (d) Application of statistical methods which have led to the formulation of the law of ancestral heredity." We shall take these up in order.