AUGUST WEISMANN 341 



cells do not arise immediately from one another as complete cells, but 

 only as minute particles of germ-plasm. This latter substance, how- 

 ever forms the foundation of the germ-cells of the next generation, 

 and stamps them with their specific character. Previous to the pub- 

 lication of my theory, C. Jager, and later M. Nussbaum, have ex- 

 pressed ideas upon heredity which come very near to my own. Both 

 of these writers started with the hypothesis that there must be a di- 

 rect connection between the germ-cells of succeeding generations, and 

 they tried to establish such a continuity by supposing that the germ- 

 cells of the offspring are separated from the parent germ-cell before 

 the beginning of embryonic development, or at least before any his- 

 tological differentiation has taken place. In this form their suggestion 

 cannot be maintained, for it is in conflict with numerous facts. A 

 continuity of the germ-cells does not now take place, except in very 

 rare instances ; but this fact does not prevent us from adopting a 

 theory of the continuity of the germ-plasm, in favour of which much 

 weighty evidence can be brought forward. In the following pages I 

 shall attempt to develop further the theory of which I have just given 

 a short account, to defend it against any objections which have been 

 brought forward, and to draw from it new conclusions which may 

 perhaps enable us more thoroughly to appreciate facts which are 

 known, but imperfectly understood. It seems to me that this theory 

 of continuity of the germ-plasm deserves at least to be examined in all 

 its details, for it is the simplest theory upon the subject, and the one 

 which is most obviously suggested by the facts of the case, and we 

 shall not be justified in forsaking it for a more complex theory until 

 proof that it can be no longer maintained is forthcoming. It does not 

 presuppose anything except facts which can be observed at any mo- 

 ment, although they may not be understood, — such as assimilation, or 

 the development of like organisms from like germs ; while every other 

 theory of heredity is founded on hypotheses which cannot be proved. 

 It is nevertheless possible that continuity of the germ-plasm does not 

 exist in the manner in which I imagine that it takes place, for no one 

 can at present decide whether all the ascertained facts agree with 

 and can be explained by it. Moreover, the ceaseless activity of re- 

 search brings to light new facts every day, and I am far from main- 

 taining that my theory may not be disproved by some of these. But 

 even if it should have to be abandoned at a later period, it seems to 



