OTHER THEORIES OF SPECIES-FORMING. 19? 



determinant for every cell that is to develop in the body, but 

 only one for every different kind of cell ; all cells exactly 

 alike can be supplied with similar determinants by the multi- 

 plication of the proper kind. Now Weismann's theory of 

 germinal selection rests upon the assumption of a competi- 

 tion or "struggle" of the determinants in the germ-plasm 

 for food and hence for opportunity to grow, to be vigorous, 

 and to multiply. The germ-cells derive their food, as do 

 the other cells and tissues of the body, from the general 

 food streams circulating around and through the cells. 

 Weismann, recognising the absolute principle of slight varia- 

 tion everywhere in Nature, it is practically impossible to 

 conceive of identity, believes that the initially slightly 

 stronger or more capable determinants will be able to take 

 up larger supplies of food, even to the extent of lessening 

 the supply for neighbouring determinants, perhaps to the 

 degree of starvation. Indeed he suggests a reason for the 

 initial slight variations in vigour of the determinants in the 

 probability that the food will reach the various determinants 

 in slightly, purely fortuitously, variable quantity, so that 

 the first inequality in vigour of the determinants will depend 

 on the fortuitous variability of food supply, while there- 

 after the variability in the determinants thus produced will 

 enable the stronger ones to draw to themselves or take up 

 more food and thus accumulate determinately the initial 

 fortuitous inequality. 



Thus when the germ-cell begins its development into 

 a new individual those kinds of cells, tissues, and organs 

 will be best developed whose determinants were most suc- 

 cessful in the struggle for food, while other parts of 

 the body may be made smaller or even may not appear 

 at all because of the starvation of the determinants re- 

 sponsible for the cells which should compose them. Also 

 these better-developed, larger, more vigorous determinants 

 of one generation will hand on to the germ-plasm of the 



