THEORIES OF HEREDITY 79 



would be capable of more than one mode of development, 

 which would explain the contradictory results of the experi- 

 ments. Furthermore, Hertwig himself, as already hinted 

 at, admits that, even from liis point of view, " the process 

 of development requires the assumption of the existence of 

 different kinds of germinal material in different kinds of 

 organisms. . . ." " But," he goes on to say, " only such 

 characters are to be ascribed to the germinal substance as 

 are appropriate to the true nature of a cell, but not those 

 numerous characters which come into existence only by 

 the interrelation of many cells and the action of environ- 

 ment." Seeing that the nature of each individual cell is 

 thus made dependent on the quality of the germ-plasm, 

 there seems to be, after all, not much difference between 

 Hertwig's and Weismann's views. 



{b) Germinal Selection. 



We are already familiarized with Weismann's idea of the 

 struggle of the determinants, having seen how he uses the 

 idea of the overpowering preponderance of one kind of de- 

 terminants over others to explain the various modes of 

 inheritance. In the cases mentioned, the struggle takes 

 place in the fertilized ovum during the process of develop- 

 ment between two sets of determinants — those derived 

 from the father, and those derived from the mother respec- 

 tively. But Weismann has extended his idea of the 

 struggle of determinants still further — to the contents of the 

 germ-cells, even before their maturation and fertilization. 

 The determinants combined in the germ-plasm of the 

 various germ-cells of the individual are subject to fluctua- 

 tions in the food-supply. Indeed, as the germ-plasm re- 

 served for the germ-cells has to grow intensively in order 

 to supply material for the countless young germ-cells of 

 the organism, the determinants of the germ-plasm grow 

 and multiply rapidly. It cannot be assumed that the nutri- 

 tive stream will be distributed absolutely evenly to all 



