74 THE METHOD OF DARWIN. 



poor; others were transferred from the hot- 

 house to the coldest part of the greenhouse; 

 all to test the relative vigor of the crossed and 

 self-fertilized seedlings. 



He considered the possible sources of error, 

 and showed that accidental cross-fertilization 

 of plants intended for self-fertilization, and 

 accidental self-fertilization of plants intended 

 for cross-fertilization, would diminish rather 

 than exaggerate his results. It having been 

 said that an excess of pollen was injurious, two 

 sets of sixty-four each of Ipomcea purpurea 

 were tested to find whether the quantity of 

 pollen applied to the stigma made any differ- 

 ence, and the statement was proved untrue. 

 He published the details of his experiments, 

 because they extended over eleven years and 

 "are not likely soon to be repeated." When 

 Darwin said in his conclusion that cross-fertil- 

 ized plants are superior to self-fertilized plants 

 and have a permanent advantage over them in 

 the struggle for existence, and that nature 

 abhors perpetual self-fertilization, there was 

 no man to gainsay it; his ingenuity had spent 

 itself in exhausting the conditions of the prob- 

 lem. He could have reached the same conclu- 

 sion from his two accidental observations and 

 his first direct experiment on the two plants. 



