168 THE CONTINUITY OF THE GERM-PLASM AS THE 



the individual, but they are derived directly from the parent germ- 

 cell. 



I believe that the latter view is the true one : I have expounded 

 it for a number of years, and have attempted to defend it, and to 

 work out its further details in various publications. I propose to 

 call it the theory of ' The Continuity of the Germ-plasm,' for it is 

 founded upon the idea that heredity is brought about by the trans- 

 ference from one generation to another, of a substance with a defi- 

 nite chemical, and above all, molecular constitution. I have called 

 this substance ' germ-plasm,' and have assumed that it possesses 

 a highly complex structure, conferring upon it the power of de- 

 veloping into a complex organism. I have attempted to explain 

 heredity by supposing that in each ontogeny, a part of the specific 

 germ-plasm contained in the parent egg-cell is not used up in the 

 construction of the body of the offspring, but is reserved unchanged 

 for the formation of the germ-cells of the following generation. 



It is clear that this view of the origin of germ-cells explains the 

 phenomena of heredity very simply, inasmuch as heredity becomes 

 thus a question of growth and of assimilation, the most funda- 

 mental of all vital phenomena. If the germ-cells of successive 

 generations are directly continuous, and thus only form, as it were, 

 different parts of the same substance, it follows that these cells 

 must, or at any rate may, possess the same molecular constitution, 

 and that they would therefore pass through exactly the same stages 

 under certain conditions of development, and would form the same 

 final product. The hypothesis of the continuity of the germ-plasm 

 gives an identical starting-point to each successive generation, and 

 thus explains how it is that an identical product arises from all of 

 them. In other words, the hypothesis explains heredity as part of 

 the underlying problems of assimilation and of the causes which act 

 directly during ontogeny: it therefore builds a foundation from 

 which the explanation of these phenomena can be attempted. 



It is true that this theory also meets with difficulties, for it 

 seems to be unable to do justice to a certain class of phenomena, viz. 

 the transmission of so-called acquired characters. I therefore gave 

 immediate and special attention to this point in my first publi- 

 cation on heredity 1 , and I believe that I have shown that the 



1 Weismann, ' Ueber die Vererbung.' Jena, 1883; translated in the present volume 

 as the second essay ' On Heredity.' 



