Environment and Adaptation 233 



cles, from which they must be forcibly removed by 

 the agency of the insect which visits the flower for 

 the nectar. As the insect enters the flower, the 

 cover of this receptacle is ruptured and the packet 

 of pollen is withdrawn and adheres firmly to the 

 head or tongue of the insect. After the pollen mass 

 is withdrawn, it often shifts its position so that it 

 will come into contact with the stigmatic surface 

 of the next flower visited. In this case, as in most 

 flowers with deep nectaries in the form of a long 

 spur, the honey can only be extracted by insects 

 with long tongues, like butterflies and bees, and the 

 extraordinary mouth parts of these insects are be- 

 yond any question adaptations for feeding upon the 

 nectar of flowers having these deep nectaries. 



Pollination of Yucca. One more example must 

 suffice, as perhaps the most peculiar adaptation to 

 cross-pollination that has yet been studied. In the 

 warmer parts of the United States there are several 

 species of the genus Yucca, comprising a number 

 of showy lily-like plants, of which some are not un- 

 common in gardens. In nearly all of the species 

 that have been studied there has been found a most 

 extraordinary case of special adaptation, these plants 

 usually being dependent for pollination upon a sin- 

 gle species of moth, of the genus Pronuba. The first 

 species which was described, P. Yuccasella, pol- 

 linates the common species of Yucca, Y. tilamentosa, 

 in the Southeastern United States. The larvae of 

 these little moths feed upon the young seeds of the 



