Origin of Different Kinds of Adaptations 367 



employed, can be of no service at all. Neither can this re- 

 lation have any connection with the facility for self-fertiliza- 

 tion. "We are led, therefore, to conclude that the rule of 

 increased sterility in accordance with increased inequality 

 in length between the pistils and stamens is a purposeless 

 result, incidental on those changes through which the species 

 has passed in acquiring certain characters fitted to insure 

 the legitimate fertilization of the three flowers." 



In regard to the plants that were raised from the seeds 

 from legitimate and illegitimate unions, Darwin found in 

 Lythrum that of twelve illegitimate unions two were com- 

 pletely barren, and nearly all showed lessened fertility ; only 

 one approached complete fertility. Darwin lays much em- 

 phasis on the close resemblance in the sterility of the illegiti- 

 mate unions, and the sterility of different species when 

 crossed. In both cases every degree of sterility is met with, 

 "from very slightly lessened fertility to absolute barrenness." 

 The importance of this comparison cannot, I think, be over- 

 estimated, for, if admitted, it indicates clearly that the infer- 

 tility between species cannot be used as a criterion of their 

 distinctness, because here, in individuals belonging to the 

 same species, we find sterility between pistils and stamens 

 of different lengths. If, as I shall urge below, we must con- 

 sider these different forms of Primula the results of a muta- 

 tion, and not the outcome of selection as Darwin supposed, 

 then this relation in regard to infertility becomes a point of 

 great interest. 



This brings us to the central point of our examination 

 of these cases of dimorphism and trimorphism. How have 

 these forms arisen ? Darwin tries to account for them as 

 follows : Since heterostyled plants occur in fourteen different 

 families of plants, it is probable that this condition has been 

 acquired independently in each family, and " that it can be 

 acquired without any great difficulty." The first step in the 

 process he imagines to have been due to great variability 



