124 THE THEORY OF MUTATION 



composite plants e.g., the daisy and chrysanthemum. 

 It is suggested that meristic variations are connected 

 with definite changes in the mechanical relations of 

 dividing parts, and that it is in the mechanics of cell- 

 division that the explanation of their discontinuous 

 appearance is to be sought for. 



Thus when, for example, a tulip-flower appears 

 having its parts perfectly developed in sets of four 

 instead of in sets of three, it is suggested that the 

 arrangement in fours, like the arrangement in threes, 

 fulfils certain conditions of equilibrium among the 

 forces which affect the cell-divisions in the rudiment 

 of the flower, and that these conditions of stability 

 would not be equally well provided for by any inter- 

 mediate arrangement. 



Substantive variations are changes in the actual 

 constitution or substance of the parts themselves. 

 For example, a plant with coloured flowers may give 

 rise to offspring the flowers of which are white. There 

 seems to be no mechanical necessity for such varia- 

 tions to be discontinuous rather than continuous ; it 

 is quite possible to imagine a gradual dilution of colour 

 taking place throughout a long series of generations. 

 Discontinuous substantive variations are, however, not 

 infrequent, and in such cases it is suggested that they 

 may be associated with definite changes in chemical 

 composition. Thus, for example, definite alterations 

 in the colour of offspring as compared with their 

 parents seem almost necessarily to be of this nature. 



The further evidence contained in the book we are 

 considering refers entirely to meristic variation. 



