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 
