VARIATION AND HEREDITY 



205 



in the material or substance of the organs and are 

 therefore called substantive (Bateson). On the other 

 hand, the number of parts may vary, as with flower- 

 petals, or the joints of an appendage, or body- 

 segments (as of a worm). Such variation is termed 

 meristic. It is clear that there can be no inter- 

 mediate between a three-leaf clover and a four-leaf 

 clover. The fourth leaf is a perfect leaf, no matter 



A--- 



FIG. 75. Rhinoceros beetles (Xylotrupes gideon) ; I, "high" male; 

 II, "low" male; A, the cephalic horn. The legs are omitted for the 

 sake of clearness. (After Bateson.) 



how small it may be. Meristic variation differs, 

 then, from substantive variation in being discon- 

 tinuous instead of continuous. 



Discontinuity of variation, however, may be dis- 

 covered in substantive as well as meristic characters. 

 When we plot a curve of measurements for a char- 

 acter, we sometimes find that it apparently shows two 

 modes, or is "double-humped." (See figs. 75, 76.) 

 Such a curve is impossible to analyze by the usual 



