368 Evolution and Adaptation 



in the length of the pistil and stamens, or of the pistil alone. 

 Flowers in which there is a great deal of variation of this 

 sort are known. "As most plants are occasionally cross- 

 fertilized by the aid of insects, we may assume that this was 

 the case with our supposed varying plant ; but that it would 

 have been beneficial to it to have been more regularly cross- 

 fertilized." " This would have been better accomplished if the 

 stigma and the stamens stood at the same level ; but as 

 the stamens and pistil are supposed to have varied much 

 in length, and to be still varying, it might well happen that 

 they could be reduced much more easily through natural 

 selection into two sets of different lengths in different indi- 

 viduals than all to the same length and level in all individuals." 

 By means of these assumptions, improbable as they may 

 appear, Darwin tries to explain these cases of dimorphism. 

 But when we attempt to apply the same argument to the 

 trimorphic forms, it is manifestly absurd to pretend that three 

 such sharply defined types could ever have been formed as 

 the result of natural selection. But we have not even yet 

 touched the chief difficulty, as Darwin himself points out. 

 " The essential character of a heterostyled plant is that an 

 individual of one form cannot fully fertilize, or be fertilized 

 by, an individual of the same form, but only by one belonging 

 to another form." This result Darwin admits cannot be ex- 

 plained by the selection theory, for, as he says, " How can 

 it be any advantage to a plant to be sterile with half of its 

 brethren, that is, with the individuals belonging to the same 

 form?" He concludes that this sterility between the indi- 

 viduals of the same form is an incidental and purposeless 

 result. " Inner constitutional differences " between the 

 individuals is the only suggestion that is offered to account 

 for the phenomenon. In other words, it is clearly apparent 

 that the attempt to apply the theory of selection has here 

 broken down, and it is a fortunate circumstance that the 

 Lamarckian theory cannot here be brought to the rescue, as 



