THE ORIGIN OF FLOWERS 



201 



conclusion is not correct, for the bees are well accustomed in many 

 flowers to find that the nectar has already been taken by other be< 

 they could therefore not conclude from one unsuccessful visit that 

 the Cypripedium did not produce nectar at all, but would try again 

 in a second, a third, and a fourth flower. If these orchids had 

 abundantly covered flower-spikes like many species of Orchis, and if 

 the species were common, the bees would probably soon learn not to 

 visit them, but the reverse is the case. There is usually only one or, 

 at most, two open flowers on the lady's slipper, and the plant is rare, 

 and probably occurs nowhere in large numbers. 



If we could find a flower in which the nectar lay open and acces- 

 sible to all insects, and which did not require any service from them in 

 return, the case could not be interpreted in terms of natural selection : 

 but we do not know of any such case. 



Conversely, too, there are no adapta- 

 tions in the insects which are useful 

 only to the flowers, and which are not 

 of some use, directly or indirectly, to 

 the insect itself. Bees and butterflies 

 certainly carry the pollen from one 

 flower to the stigma of another, but 

 they are not impelled to do this by 

 a special instinct ; they are forced to do 

 it by the structure of the flower, which 

 has its stamens so placed and arranged 

 that they must shake their pollen over 

 the visitor, or it may be that the anthers 

 are modified into stalked, viscid pollinia 

 which spring off at a touch, and fix 

 themselves, so to speak, on the insect's head. And even this is not all 

 in the case of the orchis, for the insect would never of its own accord 

 transfer these pollinia on to the stigma of the next flower; this is 

 effected by the physical peculiarity which causes the pollinia, after 

 a short time, to bend forwards on the insect's head. 



All this fits in as well as possible with the hypothesis: how 

 could an instinct to carry pollen from one flower to the stigma <»l 

 another have been developed in an insect through natural selection, 

 since the insect itself has nothing to gain from this proceeding? 

 Accordingly, we never find in the insect any pincers or any kind of 

 grasping organ adapted for seizing and transmitting the pollen. 



There is, however, one very remarkable case in which this 

 appears to be so, indeed really is so, and nevertheless it is not 



Fig. 51. The Yucca-moth /'/ - 



nuba yuccasella). M, laying eggs in 

 the ovary of the Yucca flower. 

 n, the stigma. After Riley. 



