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 
