200 DARWINISM AND THE PROBLEMS OF LIFE 



during the night. In fact, many plants that are only- 

 visited by hawk-moths only give out their perfume at 

 night when these moths fly. Many will have already 

 seen this in the case of the caprifoliacese. 



At first sight it would look as if those flowers were in 

 the best position in which the honey was most exposed, 

 and which were visited by as many insects as possible. 

 But that is not the case. In the first place, many insects 

 are so small that they can eat the honey without rubbing 

 against the pollen, and so they are useless to the flowers ; 

 and in the second place, the flower will have a much 

 better chance of being fertilised if only a few species of 

 insects visit it. When a certain kind of flower is liked 

 by a particular insect, this will be the more likely to 

 return to it, and not waste its pollen on other flowers. 

 On this account the honey that was at one time exposed 

 in many flowers has sunk deeper into them, and thus 

 can only be reached by the more intelligent insects. 

 The effect is enhanced by the curling of their leaves so 

 as to form a tube which varies in thickness, and so 

 admits different kinds of insects. In some flowers the 

 tube is so narrow that it takes the long proboscis of a 

 butterfly to reach the honey. Others have adapted 

 themselves to flies, and give out the smell of carrion, 

 which attracts these alone. In the aristolochia the long 

 and narrow tube is further provided with hairs on all 

 sides that point downwards ; these let the fly in, but 

 prevent it from escaping. The insect is kept captive 

 until it is covered with dust by the stigma at the 

 bottom, when the hairs wither, and it can get away. 

 In this way the restless fly is compelled to do its duty. 



