BEES DO THEIR WORK 335 



ing desert air the stigma of the yucca, if not 

 placed deep within the tissues, could hardly en- 

 dure exposure and still perform its function. 

 The arrangement between stamens and pistil 

 of the yucca is such that no other insect is 

 likely to pollenize it, even were there other insects 

 at hand. 



Altogether this is one of the most curious and 

 thought-provoking instances in all nature of 

 mutual dependence between an animate creature 

 and a plant. 



One can scarcely leave the yucca and its 

 strange visitor without inquiring how so ex- 

 traordinary a coalition could have been brought 

 about. Unfortunately no very precise answer 

 can be supplied. We can only assume that the 

 complex and intricate relationship now mani- 

 fested is the final result of a long series of slight 

 adaptations through which insect and plant were 

 mutually specialized in such a way as to conform 

 to each other's needs. 



It is impossible to conceive that any sudden 

 mutation of form on the part of the plant or of 

 habit on the part of the insect could have led to 

 so complicated an alliance. 



The change must have been very slow and 

 gradual. First, we may suppose a condition in 

 which the ancestors of the yucca were sometimes 



