262 THE GERM-PLASM 



corolla-tube, on the other hand, the hybrid inclines more to iV. 

 rustica^ while in the narrowest part it exhibits the exact mathe- 

 matical mean. This case is instructive, for we cannot recognise 

 the true physiological mean, because the length, as well as the 

 diameter, of the corolla-tube is determined by the same cells. 

 Any estimation of the mean between the colours must be still 

 less precise, for the different shades depend on entirely dif- 

 ferent morphological constituents. If the yellow and red of two 

 different species were blended in the flower of a hybrid, the 

 intensity of both these colours might conceivably be as great as 

 in the parent plant, and yet one of them might predominate 

 because it happened to cover the other. F'or the yellow is due 

 to special pigment-granules, while the red occurs onlv in the 

 cell-sap, which might possibly be nearly hidden by a superficial 

 layer of chromatophores. 



In any inquiry with regard to the factors which control the 

 struggle of the parental characters, it must, above all, be 

 borne in mind that tJie cells are always the deter)ni}ii}ig agents. 

 The determinants of the father and mother come together in the 

 cell, and in the cell only : and all characters, whether relating to 

 a large part of the organism, or merely to a single cell, can only 

 be determined by processes taking place within the substance of 

 one or of many cells. 



This does not mean that the visible differentiation of every 

 individual cell always constitutes a 'character' of the organism. 

 The nature of the histological differentiation of the cells — that is 

 to say, whether muscle- or nerve-substance, or chlorophyll 

 granules, for instance, are produced in the cell-body or not — 

 only comes into consideration in connection with the lumiber of 

 cells in the definitive cell-aggregate constituting the organism. 

 Very many characteristic qualities cannot be due to this fact, 

 but must chiefly depend on the number and arrangement of the 

 cells in an organ, which again must be due to qualities of the 

 embryonic cell which are invisible to us — principally those which 

 relate to its method of tiivision^ and its vigour and rate of multi- 

 plication. 



We must suppose that these factors are wholly determined by 

 the idioplasm of the cells quite as much as is the visible differen- 

 tiation of the latter. The division of a cell certainly originates 

 in its apparatus for division, and primarily in the ' sphere of 

 attraction ' and its contained centrosome ; but we should ha\e to 



