40 



THE AMERICAN NATURALIST [Vol. LI1 



may be ineffective, if a modification only is selected; it 

 will be partly successful if a combination of plus-quantity 

 with plus-modification is selected; and fully successful 

 if the exclusive result of plus-quantity of the gene is se- 

 lected. The dens ex machina modifying factor, which, 

 moreover, does not fit the decisive genetic facts in the 

 most discussed case of Castle's rats nor our cases, thus 

 becomes superfluous. 



It is, moreover, perfectly logical to assume that selec- 

 tion of either plus or minus quantities of the genes 

 changes the mode of the fluctuation of this quantity corre- 

 spondingly in the succeeding generation. If the different 

 quantities of the substances, which constitute the systems 

 of multiple allelomorphs, are inherited, then every other 

 quantity is also inherited. If the presence of the quan- 

 tity p in the germ cells of the parents causes the reap- 

 pearance of the quantity p in the germ cells of the chil- 

 dren, the same fact applies to the quantities q, r, s— to 

 every quantity which is present or has been selected. Se- 

 lection can, therefore, change the quantity of the gene, 

 and also, therefore, the somatic characters caused by 

 quantitative differences in the gene, until the physiolog- 

 ical limit is reached. This limit may be the limit for the 

 character in question— for example, no pigment, self- 

 color— or it may be the limit set by the necessary coor- 

 dination of developmental processes. For example, in 

 the development of a moth a certain gene causes, at a cer- 

 tain moment-during pupation-the evagination of the 

 imaginal disks of the antenna. The correct quantity of 

 the gene causes this process to take place at the correct 

 time. A quantitative variation of the gene would cause 

 the evagination to take place at the wrong time. We 

 have, indeed, had strains of caterpillars where in many 

 individuals this process took place in the last stage of the 

 caterpillar, giving caterpillars with pupal antenna. The 

 quantity of the gene in question was in these cases not co- 

 ordinated with the other genes and the action was pro- 

 duced too early. It is evident that quantitative changes 



