THE ORIGIN OF FLOWERS 



185 



Very remarkable are those flowers which are adapted to the visits 

 of flies, for they correspond in several respects to the peculiarities of 

 these insects. In the first place, flies are fond of decaying substances 

 and the odours given off by these, and so the flowers which depend 

 for their cross-fertilization on flies have taken on the dull and ugly 

 colours of decay, and give out a disagreeable smell. But flies are also 

 shy and restless, turning now hither, now thither, and cannot be 

 reckoned among the ' constant ' insect visitors, that is to say, they 

 do not persistently visit the same species ; it is, therefore, evident that 



Fig. 44. Alpine Butter- 

 wort {pinguicula alpina). 

 ^ , section tlirough the flower. 

 K, calyx. 6ft, bristly promi- 

 nences, sp, spur, st, stamen, 

 m, stigma. B, -stigma and 

 stamen Inore magnified. 

 After H. Miiller. 



Fig. 43. Flower of Birthwort(Jlris- 

 tolochia clemaiitis) cut in half. A, be- 

 fore pollination by small flies. 6, the 

 bristles. JB, after pollination. P, pol- 

 len mass. N, stigma. 6, the bristles. 

 6', their remains. After H. Miiller. 



they might easily carry away the pollen without any useful result 

 ensuing. Moreover, their intelligence is of a low order, and they do 

 not seek nectar with the perseverance shown by bees and humble-bees. 

 It is not surprising, therefore, to find that many of the flowers adapted 

 for the visits of flies are so constructed that they detain their visitors 

 until they have done their duty, that is to say, until they have efiected, 

 or at least begun, the process of cross-pollination. 



Our birthwort (Aristolochia clematitis) and the Cuckoo-pint 

 {Arum maculatum) are pit-fall flowers, whose long corolla-tubes have 



