NOCTURNAL OR HAWK-MOTH FLOWERS 



cessity of building nests and caring for their young made this 

 impossible in the case of bees. Just as the momentum of a 

 swinging pendulum carries it beyond the central point of equi- 

 librium, so the momentum of the variation carried the length 

 of the nectaries to a point where bees were excluded. Thus 

 butterfly and moth flowers came into existence; but this re- 

 ciprocal dependence does not in most cases imply more effec- 

 tive pollination, although the Lepidoptera are assured a larger 

 supply of nectar. Tendency to vary in a definite direction, 

 even when no benefit is derived, is shown by the increasing 

 complexity of the markings on the wings of insects and the 

 convolutions on the margins of shells. It is not at all improb- 

 able that the nectaries of certain flowers and the tongues of 

 the visiting Lepidoptera may continue to lengthen until both 

 become extinct. 



An intricate mechanism to effect pollination does not prove 

 that such an arrangement is best for flowers in general. It 

 finds its explanation in the particular conditions under which 

 each flower was developed. Nature seems at times to be a 

 very poor teleologist. The wonderful orchids are less success- 

 ful than many lowly dooryard weeds. In the case of certain 

 flowers orthogenesis, or determinate variation in a definite 

 direction, has carried forward their specialization until they 

 are face to face with extinction. 



159 



