HEREDITY. 15 



When we take a biomechanical view of inheritance we have to 

 look into this matter. To ascribe the qualities of the cells to 

 qualities of genes, and then to turn round and state that these 

 genes are protoplasm, is nothing but deferring the difficulty 



To ascribe a vital nature to the genes admits of explaining 

 their variability ; this must be the fundamental reason under- 

 lying the construction of these theories. As Dewar and Finn 

 frankly state : These biological molecules have all the proper- 

 ties of living matter, including variability. And Weissmann's 

 hypothesis of germinal selection, of a struggle for the available 

 nourishment between the determinants, and de Vries' idea of 

 latent and semi-latent and labile states of pangens could not be 

 held, should the vital nature of the genes themselves be given 

 up. 



Since the time of Darwin, ever since Mendel's work got 

 known, we have learned a great many facts about genes. To sura 

 up : We know, that they are inherited from mother-cells to 

 daughter-cells, but do not pass from cell to cell (Periklinal 

 chimera's). We known that genes, which are inherited in only 

 one gamete, will later be furnished to one half the number of 

 gametes produced (Mendel's law), but that the influence upon 

 the development in such a case is fully or approximately as 

 great as in the case, where both gametes that make up the 

 zygote contain the gene. We know, that the genes must have 

 a nature which admits of their quantitative multiplication, but 

 we also know, that the genes themselves are qualitatively 

 stable and non- variable (Johannsen's law). 



A vitalistic view of the nature of the genes certainly fits the 

 facts, but whereas it is a theory that will work, it is not a 

 theory that one can work with. The main new fact we know 

 about genes is, what we have called Johannsen's law. It has 

 been shown, conclusively we think, that inheritable variability 

 is synonymous with genotypic impurity. For as far as a group 

 of organisms contains some which have, and others which do 

 not have certain genes, or some which are impure, (heterozygous) 

 for one or more genes, this group has, what we want to call by 



