466 THE UNIVERSE. 



winters of these polar regions, alone preserve all the popu- 

 lation from famine. 



Willdenow, by means of an interesting experiment, 

 showed plainly what a part insects play in respect to fructi- 

 fication. He took an Aristolochia Clematitis and placed it 

 under a cage covered with gauze. As this prevented the 

 animals from reaching and penetrating within the flowers, 

 the plant produced no fruit. On the other hand, another 

 Aristolochia of the same species, which stood by the side of 

 it in the open air, so that the insects could visit it as they 

 liked, had all its flowers fecundated. 



The idea of the intervention exercised by insects is so 

 predominant with Burdach that he goes the length of sup- 

 posing that each nourishes its particular insect, the mission 

 of which is to preside over the mysteries of its espousal. 

 According to the German physiologist, flowers only pre- 

 serve their original purity because their faithful visitors 

 consecrate the whole course of their ephemeral existence 

 to them, and never wander to another species. The noc- 

 turnal plants are also haunted by useful parasites, which 

 only awake to animation during the darkness. 1 



Conrad Sprengel even thinks that if so many flowers are 

 stricken with sterility in our hot-houses, even when parad- 

 ing a superfluity of means for becoming mothers, it is be- 

 cause their indispensable insect has not been allowed to 

 bear them company. This is the case with the Vanilla. 



1 Dr. Hildebrand, of Bonn, concludes, from several interesting experiments on 

 the fertilization of Corydalis cava, that when the flowers of the plant are pro- 

 tected from insect influence, and acted on only by their own pollen, they produce 

 no capsules. 



