106 THE METHOD OF DARWIN. 



crossed, and of their hybrid offspring, is due 

 to functional differences of the sexual elements, 

 and not to structural differences between the 

 species, that led him to make the many experi- 

 ments on the illegitimate offspring of hetero- 

 styled plants, and that made the results worthy 

 of publication. The great strength of the 

 analogy in his mind was doubtless due to the 

 fact that the parallelism was so very close as to 

 force the conclusion that the two sets of results, 

 one within a species, the other between species, 

 are due to the same cause. 



