XUlvery hypothesis from which it derives its name is untenable.There is now scarcely any doubt that the entire conception of the production of the ' gemmules ' by the body-cells, their separation from the latter, and their ' circulation,' is in reality wholly imaginary.In this regard I am still quite as much opposed to Darwin's views as formerly, for I beheve that all parts of the body do not contribute to produce a germ from which the new individual arises, but that, on the contrary, the offspring owes its origin to a peculiar substance of extremely complicated structure, viz., the * germ-plasm.'This substance can never be formed anew ; it can only grow, multiply, and be transmitted from one generation to another.My theory might therefore well be denominated *" bias to -gene sis ' or origin from a germ-plasm, in contradistinction to Darwin's theory of^pangenesis ' or origin from all parts of the body.My doubts as to the validity of Darwin's theory were for a \, long time not confined to this point alone : the assumption | of the existence oi p7'eformed constituents of all parts of the body seemed to me far too easy a solution of the difficulty, \ besides entailing an impossibility in the shape of an abso-| lutely inconceivable aggregation of primary constituents. * The theory of * evolution ' or * preformation ' of the early physiologists supposed that all parts of the fully-formed animal or plant were present, in a minute form, in the germ. The rival theory of * epigenesis ' XVlll PREFACE during two winter sessions. My hearty thanks are also due to my friends and colleagues Professors Baumann, Liiroth, Wiedersheim, and Ziegler, as well as to Professor Goebel, of Munich, for information of various kinds ; and I am no less indebted to Miss Else Diestel, who, in addition to much help of a technical nature, has also been at the great pains of preparing an alphabetical index. I thus venture to bring into the light of day a work which is the fruit of many years labour and of many doubts ; and even though but few of my results should remain unmodified, I hope nevertheless that my work has not been in vain ; for even error, if it originate in correct deductions, must become a step towards truth. AUGUST WEISMANN. Freiburg, i/Br., May igth 1892. 20. Formation of spermatozoa in Ascaris viegalocephala. 21. Formation of ova in Ascaris megalocephala. 22. Diagram to illustrate the combination of idants in hybriils. 23. The two varieties of Cypris reptans. 24. Bonellia viridis, male and female. xxiii * ' Wiener Sitzungsberichte,' Oct. lo, 1861,Bd. 44, ii., p. 381, ') of one portion of the organism. I must honestly confess to having mentally resisted this fundamental point of the Darwinian doctrine for a long time. It appeared almost impossible to me that such an enormously large number of individual primary constituents as we must suppose to exist, according to Darwin's view, could be contained in the minimum of substance which, as will be shown hereafter, w^ehave to regard as the actual bearer of heredity. 1 tried in sever...
