164 Evolution and Adaptation 



Weismann continues : " But the question remains, why is 

 this the fact ? " He believes his hypothesis of the existence 

 of determinants in the germ gives a satisfactory answer to 

 this " why." " According to this theory every independent 

 and hereditarily variable part is represented in the germ by 

 a determinant, whose size and power of assimilation corre- 

 sponds to the size and vigor of the part. These determinants 

 multiply as do all vital units by growth and division, and 

 necessarily they increase rapidly in every individual, and the 

 more rapidly the greater the quantity of the germinal cells 

 the individual produces. And since there is no more reason 

 for excluding irregularities of passive nutrition, and of the 

 supply of nutriment in these minute, microscopically invisible 

 parts, than there is in the larger visible parts of the cells, 

 tissues, and organs, consequently the descendants of a deter- 

 minant can never all be exactly alike in size and capacity of 

 assimilation, but they will oscillate in this respect to and fro 

 about the maternal determinant as about their zero point, and 

 will be partly greater, partly smaller, and partly of the same 

 size as that. In these oscillations, now, the material for 

 further selection is presented, and in the inevitable fluctu- 

 ations of the nutrient supply, I see the reason why every 

 step attained immediately becomes the zero point of new 

 fluctuations, and consequently why the size of a part can 

 be augmented or diminished by selection without limit, solely 

 by the displacement of the zero point of variation as the 

 result of selection." 



The best illustration of this process of germinal selection 

 is found, Weismann believes, in the case of the degeneration 

 of organs. " For in most retrogressive processes active selec- 

 tion in Darwin's sense plays no part, and advocates of the 

 Lamarckian principle, as above remarked, have rightly 

 denied that active selection, that is, the selection of indi- 

 viduals possessing the useless organ in its most reduced 

 state, is sufficient to explain the process of degeneration. I, 



