Since the great problem of heredity was forced by Darwin in 1859 into the foreground of general biology, many different hypotheses and theories of it have been framed. All these have in the end to trace it to molecular features in the plasm of the germ-cells; because it is this germ-plasm of the maternal ovum and the paternal sperm-cell that conveys the characteristics of the parents to the child. Hence the great progress that has been made recently in the study of conception and heredity, by means of a number of remarkable observations and experiments, has been of service to our ideas on the molecular structure of the plasm. I have dealt with the chief of these theories in the ninth chapter of my _History of Creation_, and must refer the reader thereto. In chronological order we have: (1) the pangenesis theory of Darwin (1868), (2) the perigenesis theory of Haeckel (1875), (3) the idioplasm theory of Nägeli (1884), (4) the germ-plasm theory of Weismann (1885), and (5) the mutation-theory of De Bries (1889). None of these attempts, and none of the later theories of heredity, has given us a satisfactory and generally admitted idea of the plasma-structure. We are not even clear as to whether in the last resort life is to be traced to the several molecules, or to groups of molecules, in the plasm. With an eye to this latter difference, we may distinguish the plastidule and micellar theories as two different groups of relevant hypotheses. In my essay on "The Perigenesis of the Plastidules" (1875) I formulated the hypothesis that in the last instance the plastidules are the vehicles of heredity--that is to say, plasma-molecules which have the property of _memory_. In this I found support in the ingenious theory of the distinguished physiologist, Ewald Hering, who had declared in 1870 that "memory is a general property of organic matter." I do not see still how heredity can be explained without this assumption! The very word "reproduction," which is common to both processes, expresses the common character of psychic memory (as a function of the brain). By plastidules I understand simple molecules; the homogeneous nature of the plasm in the monera (both chromacea and bacteria and rhizomonera) and the primitive simplicity of their life-functions do not dispose us to think that special groups of molecules are to be distinguished in these cases. Max Verworn has recently (1903) formulated his biogen-hypothesis in the same sense, as a "critical-experimental study of the processes in the living matter." He also takes the active plasma-molecules, which he calls biogens, as the ultimate individual factors of the life-process, and is convinced that in the simplest cases the plasm consists of homogeneous biogen-molecules.