16 



HEREDITY 



[CH. 



Another rather frequent condition is that the 

 curve has two maxima or modes (fig. 5), indicating 

 that a large number of individuals have a low measure- 

 ment, a less number are intermediate, and again a 

 larger number have a higher measurement. Species 

 which vary in this way are called ' dimorphic/ or if 



5 6 7 8 



Fig. 4. 'Half-curve* representing the number of 'petals' on 416 

 flowers of the marsh-marigold (Caltha palustris). (After de Vries. ) 



there are more than two peaks to the curve, ' poly- 

 morphic/ Further, it is possible that the two parts 

 of the curve should be entirely separate, if inter- 

 mediates between the low and high groups are 

 completely wanting. This type of variation is spoken 

 of as ' discontinuous ' in contrast to the ' continuous ' 



