240 GENETICS 



elaborate devices, a separation of the sexes, especially 

 among the higher * organisms, and in consequence 

 there has arisen an unavoidable necessity of out- 

 crossing. The intricate adaptations existing between 

 insects and flowers, for example, seem to be directed 

 entirely toward insuring out crossing among plants. 



9. EXPERIMENTS TO TEST THE EFFECT OF IN- 

 BREEDING 



Numerous experiments to test the effect of in- 

 breeding have been carried out upon various or- 

 ganisms. 



Darwin, for instance, planted morning-glories, 

 Ipomcea, derived from the same stock of seeds, in 

 two beds which were laid out side by side, that is, 

 in an environment as nearly the same as possible, 

 but with half of the beds screened from insects 

 which usually transfer pollen from flower to flower. 

 In the screened half where all insects were excluded 

 the flowers were of necessity self-fertilized, while in 

 the exposed half they were presumably cross-pol- 

 linated by the insects which had free access to them. 

 The seeds produced in the two beds were kept sep- 

 arate and the experiment was continued for ten years, 

 so that at the end of that time two lots of morning- 

 glories, one self-fertilized for ten generations and the 

 other presumably cross-pollinated for the same length 

 of time, were obtained for comparison. The crite- 

 rion Darwin used was the vigor of the plants as 

 shown by the length of the vine. He found that the 



