[V. IN THE THEORY OF NATURAL SELECTION. 273 



that is if I am right in believing that in all animals and plants 

 which are reproduced by true germs, only those characters 

 which were potentially present in the germ of the parent can 

 be transmitted to the succeeding generation. 



I believe that heredity depends upon the fact that a small 

 portion of the effective substance of the germ, the germ-plasm, 

 remains unchanged during the development of the ovum into 

 an organism, and that this part of the germ-plasm serves as 

 a foundation from which the germ-cells of the new organism 

 are produced ^ There is therefore continuity of the germ- 

 plasm from one generation to another. One might represent 

 the germ-plasm by the metaphor of a long creeping root-stock 

 from which plants arise at intervals, these latter representing 

 the individuals of successive generations. 



Hence it follows that the transmission of acquired characters 

 is an impossibility, for if the germ-plasm is not formed anew in 

 each individual but is derived from that which preceded it, its 

 structure, and above all its molecular constitution, cannot 

 depend upon the individual in which it happens to occur, but 

 such an individual only forms, as it were, the nutritive soil at 

 the expense of which the germ-plasm grows, while the latter 

 possessed its characteristic structure from the beginning, viz. 

 before the commencement of growth. 



But the tendencies of heredity, of which the germ-plasm is 

 the bearer, depend upon this very molecular structure, and 

 hence only those characters can be transmitted through suc- 

 cessive generations which have been previously inherited, viz. 

 those characters which were potentially contained in the struc- 

 ture of the germ-plasm. It also follows that those other 

 characters which have been acquired by the influence of special 

 external conditions, during the life-time of the parent, cannot 

 be transmitted at all. 



The opposite view has, up to the present time, been main- 

 tained, and it has been assumed, as a matter of course, that 

 acquired characters can be transmitted ; furthermore, extremely 

 complicated and artificial theories have been constructed in 

 order to explain how it may be possible for changes produced 



^ Compare the second and fourth of the preceding Essays, ' On 

 Heredity ' and ' The Continuity of the Germ-plasm as the Foundation of 

 a Theory of Heredity.' 



