186 MR. C. DABWIS- Oy THE SEXUAL EELATIOITS OJ? 



propriate pollen coming from the stamens of corresponding length 

 borne by the other two forms, and though the homamorpUc 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 tmions, 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 curious 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 arc 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 pistU 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 



