II.j 



DIMORPHISM. 



in the other set, on the contrary, the anthers are at the 

 mouth of the flower, and the stigma half-way down. 

 The existence of these two kinds of flowers had long 

 been known, but it remained unexplained until Mr. 

 Darwin devoted his attention to the subject. Now 

 that he has furnished us with the clue the case is 

 clear enough. 



An insect visiting a plant of the short-styled form 

 would dust its proboscis at a certain distance from the 

 extremity (Fig. 30, a], which, when the insect passed 



FIG. 29. Primula (long-styled form). FIG. 30. Primula (uhort-styled form). 



to a long-styled flower, would come just opposite to 

 the pistil (Fig. 2Q,st\ At the same time, the stamens 

 of this second form (Fig. 29, a) would dust the pro- 

 boscis at a point considerably nearer to the extremity, 

 which in its turn would correspond to the position of 

 the stigma in the first form (Fig. 30, sf). The two 

 kinds of flowers never grow together on the same stock, 

 and the two kinds of plants generally grow together 

 in nearly equal proportions. Owing to this arrange- 

 ment, therefore, insects can hardly fail to fertilise each 

 flower with pollen from a different stock. 



