GERMINAL SELECTION 125 



though with some individuality of expression. The child is tluis not 

 determined by the determinants of a single id, but by those of many 

 ids, and the variations of any part of the body do not depend on the 

 variations of a single determinant X, but on the co-operation of all 

 the determinants X which are contained in the collective ids of the 

 relevant germ-plasm. Thus it is onlj^ when a majority of the 

 determinants have varied upwards or downwards that they dominate 

 collectively the development of the part X' and cause it to be larger 

 or smaller. 



We have assumed passive tiuctuations in nutrition to be the first 

 cause in individual variation, and it is obvious that the action of this 

 first cause of dissimilarity must be greatly restricted by the multi- 

 plicity of the ids and the corresponding homologous determinants. 

 For although passive fluctuations in nutrition should occur continually 

 in the case of h\\ determinants, this would not imply that they 

 would follow the same direction in all the determinants X of all 

 ids, for some determinants X might vary upwards, and others 

 downwards, and these might counteract each other in ontogeny ; so 

 that in many cases the fluctuations of the individual determinants 

 will not be felt in their products at all. But since there are — as we 

 shall see later — only two directions of variation, upwards and 

 downwards, plus and minus, it must also sometimes happen that 

 a majority take one direction, and this affords the basis on which 

 germinal selection can build further, and on which it is materially 

 supported by reducing division and the subsequent amphimixis. 



For reducing division removes half of the ids and thus of 

 the determinants from the mature germ-cell, and according as chance 

 leaves together or separates a majority of X-determinants varying 

 in the same direction, this particular germ-cell will contain the 

 primary constituents of a plus- or of a minus-variation of X, and it 

 is possible that the presence of a majority or a minority may be 

 entirely due to the reduction. The germ-plasm of the parent may 

 contain, for instance, the determinant X in its twenty ids 12 times 

 in minus- variation form, 8 times in plus- variation form: and the 

 reducing division, according to our view, may separate these into two 

 groups of which one contains eight plus- and two minus-variations, 

 the other ten minus- variations, or the one six plus- and four minus- 

 variations, the other two plus- and eight minus-variations, and so on. 

 Now every germ-cell which contains a majority of plus- or minus- 

 variations— and this must be the case with most of them— may unite, 

 if it attains to amphimixis, with a germ-cell which also contains 

 a majority of plus or minus X^-determinants, and if similar majorities 



