170 CONTINUITY OF THE GERM-PLASM AS THE [IV. 



seems to me, only two other possible, physiologically con- 

 ceivable, theories as to the origin of germ-cells, manifesting 

 such powers as we know they possess. Either the substance 

 of the parent germ-cell is capable of undergoing a series of 

 changes which, after the building-up of a new individual, leads 

 back again to identical germ-cells ; or the germ-cells are not 

 derived at all, as far as their essential and characteristic sub- 

 stance is concerned, from the body of the individual, but the}' 

 are derived directly from the parent germ-cell. 



I believe that the latter view is the true one : I have ex- 

 pounded 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 transference from one generation to another, of 

 a substance with a definite 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 developing into a complex 

 organism. I have attempted to explain heredity by supposing 

 that in each ontogeny, a part of the specific germ-plasm con- 

 tained 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 fundamental 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 hy- 

 pothesis 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 



