424 THE GERM-PLASM 



of double determinants of single ids, as well as their purely 

 qualitative modifications, may be increased by means of the 

 ' reducing division ' and amphimixis, till the variation becomes 

 perceptible, and natural selection comes into play. 



The ' increase ' of a character derived from two parents which 

 merely possessed it to a slight extent, is an entirely different 

 matter. A ///sion of the primary constituents of a character 

 common to both parents, and a consequent increase of these 

 primary constituents, such as was hitherto supposed to take 

 place, evidently do not occur: such an assumption is contra- 

 dicted by ordinary experience ; for if the primary constituents of 

 the parents could accumulate in this way, all parts of the offspring 

 must be twice as large, or at any rate larger than, those of the 

 parent, and this is not the case. If, however, it were main- 

 tained that it is merely a question of the differences in the 

 primary constituents, and that the offspring 4-eceives half the 

 sum of the characters of the two parents, it could be replied that 

 this might be approximately true in some cases, but no expla- 

 nation would be given as to how an increase of a characteristic 

 can occur, and may even be produced artificially by pairing 

 animals which exhibit a certain tendency to vary in the desired 

 direction. If two animals are paired which possess the char- 

 acter a in the degree 2a, the offspring would contain ~~ "*" ^^ 



2 

 — i.e., 2a, and no increase would be produced. Moreover, the 

 real meaning of the term 'halving' would then remain perfectly 

 vague. Hensen * thought that ' the effect of transmission was to 

 Aalve the characters of the parents,' since 'a similar whole can 

 only result from two equal half-transmissions.' I believe that 

 this is to a certain extent true, but not in the sense which would 

 entail the halving of an indivisible primary constituent. 



The solution of tlie problem is to be found in the multiplicity 

 of the ids and determinants, and in distinguishing carefully 

 between the vague idea of the ' character ' and the definite one of 

 the determinate or hereditary part. Each determinate is con- 

 trolled by as many determinants as there are ids in the germ- 

 plasm ; but half the ids of each parent, together with half the 

 determinants of e-i'ery kind, are removed from the germ-plasm by 

 the reducing division of the germ-cell. Half the primary constit- 



* V. Hensen, ' Physiologic der Zeugung.' 



