186 MR. C. DARWIN ON THE SEXUAL RELATIONS OF 
propriate pollen coming from the stamens of corresponding length 
borne by the other two forms, and though the Aomomorphic unions 
of the females with their own two sets of males are always more 
or less sterile, there remain in each case two other sterile unions, 
not included in these two terms. Hence it will be found con- 
venient to designate the two unions of each female with the two 
sets of stamens of corresponding length, which are fully fertile, as 
legitimate unions, and the four other, more or less sterile, unions 
of each female with the four other sets of stamens as illegitimate 
unions. Consequently, of the eighteen possible unions between 
the three forms, six are legitimate and twelve are illegitimate. 
Another and eurious conclusion cannot be considered as proved, 
but is rendered highly probable, by the Tables. The unions of 
the pistils and stamens of equal length are alone fully fertile. 
Now with the several illegitimate unions it will be found that the 
greater the inequality in length between the pistil and stamens, 
the greater the sterility of the result. "There is no exception to 
this rule. Thus, with the long-styled form, its own shorter sta- 
mens are far less equal in length to the pistil than its own longer 
stamens ; and the capsules fertilized by the pollen of the shorter 
stamens yielded fewer seeds: the same comparative result follows 
from the use of the pollen of the shorter stamens of the mid-styled 
form, which are much shorter than the shorter stamens of the short- 
styled (see diagram), and therefore less equal in length to the long- 
styled pistil. We shall see exactly the same result if we look to 
the four illegitimate unions under the mid- and short-styled forms. 
Certainly the difference in sterility in these several cases is very 
slight, but the sterility always increases with the increasing 
inequality of length between the pistil and the stamens which are 
used. Therefore I believe in the above rule; but a vast number 
of artificial unions would be requisite to prove it. If the rule be 
true, we must look at it as an incidental and useless result of the 
gradational changes through which this species has passed in 
arriving at its present condition. On the other hand, the corre- 
spondence in length between the pistil of each form and those 
stamens which alone give full fertility is clearly of service to the 
species, and is probably the result of direct adaptation. 
Some of the illegitimate unions yielded, as may be seen in the 
Tables, during neither year a single seed ; but, judging from the 
case of the long-styled plant, it is probable, if such unions could 
be effected repeatedly under the most favourable conditions, some 
few seeds would be produced. Anyhow, I can state that in all 
ne eine 
