differences on the part of unicellular ancestors. And, lastly, it follows also that these congenital variations—although now so entirely independent of external conditions of life, and even of activities internal to organisms themselves—were originally and exclusively due to the direct action of such conditions on the lives of their unicellular ancestors; while even at the present day no one congenital variation can arise which is not ultimately due to differences impressed upon the protoplasmic substance of the germinal elements, when the parts of which these are now composed constituted integral parts of the protozoa, which were directly and differentially affected by their converse with their several environments. Again, if for the sake of distinctness we neglect all these far-reaching deductions from his theory of _heredity_ whereby Weismann constructs this elaborate theory of _organic evolution_, and fasten our attention only upon the former, we may briefly summarize the fundamental difference between his theory of _heredity_ and Darwin’s theory of _heredity_ thus. Darwin’s theory of heredity is the theory of _Pangenesis_: it supposes that _all_ parts of the organism _generate_ anew in every individual the formative material which, when collected together in the germ-cells, constitutes the potentiality of a new organism; and that this new organism, when developed, resembles its parents simply because _all_ the formative material in each of the parents has been thus _generated_ by, and collected from, _all_ parts of their respective bodies. Weismann’s theory of heredity, on the other hand, is the theory of the _Continuity of Germ-plasm_: it supposes that _no_ part of the parent organism generates _any_ of the formative material which is to constitute the new organism; but that, on the contrary, this material stands to all the rest of the body in much the same relation as a parasite to its host, showing a life independent of the body, save in so far as the body supplies to it appropriate lodgement and nutrition; that in each generation a small portion of this substance is told off to develop a new body to lodge and nourish the ever-growing and never-dying germ-plasm—this new body, therefore, resembling its so-called parent body simply because it has been developed from one and the same mass of formative material; and, lastly, that this formative material, or germ-plasm, has been continuous through all generations of successively perishing bodies, which therefore stand to it in much the same relation as annual shoots to a perennial stem: the shoots resemble one another simply because they are all grown from one and the same stock. CHAPTER II. LATER ADDITIONS TO WEISMANN’S SYSTEM UP TO THE YEAR 1892.