266 CONCLUDING REMARKS Chap. VI. 



the short-styled; but this is hardly ever the case. On 

 the contrary, there is sometimes the widest differ- 

 ence in this respect, as between the two illegitimate 

 unions of Pulmonaria angustifolia and of Hottonia pa- 

 lustris. 



It is a more probable view that the male and female 

 organs in two sets of individuals have been by some 

 means specially adapted for reciprocal action; and 

 that the sterility between the individuals of the same 

 set or form is an incidental and purposeless result. 

 The meaning of the term " incidental " may be illus- 

 trated by the greater or less difficulty in grafting or 

 budding together two plants belonging to distinct 

 species; for as this capacity is quite immaterial to 

 the welfare of either, it cannot have been specially 

 acquired, and must be the incidental result of differ- 

 ences in their vegetative systems. But how the sex- 

 ual elements of heterostyled plants came to differ from 

 what they were whilst the species was homostyled, 

 and how they became co-adapted in two sets of indi- 

 viduals, are very obscure points. We know that in 

 the two forms of our existing heterostyled plants the 

 pistil always differs, and the stamens generally differ 

 in length; so does the stigma in structure, the anthers 

 in size, and the pollen-grains in diameter. It ap- 

 pears, therefore, at iirst sight probable that organs 

 which differ in such important respects could act on 

 one another only in some manner for which they had 

 been specially adapted. The probability of this view 

 is supported by the curious rule that the greater 

 the difference in length between the pistils and sta- 

 mens of the trimorphic species of Lythrum and Oxalis, 

 the products of which are united for reproduction, by 

 so much the greater is the infertility of the union. 

 The same rule applies to the two illegitimate unions 



