CHAP. VI. ON HETEBOSTYLED PLANTS. 265 



acquired unless it were highly beneficial to the species : 

 and although it may be beneficial to an individual 

 plant to be sterile with its own pollen, cross-fertilisa- 

 tion being thus ensured, how can it be any advan- 

 tage to a plant to be sterile with half its brethren, 

 that is, with all the individuals belonging to the 

 same form ? Moreover, if the sterility of the unions 

 between plants of the same form had been a special 

 acquirement, we might have expected that the long- 

 styled form fertilised by the long-styled would have 

 been sterile in the same degree as the short-styled 

 fertilised by the short-styled ; but this is hardly ever 

 the case. On the contrary, there is sometimes the 

 widest difference in this respect, as between the two 

 illegitimate unions of Pulmonaria angustifolia and of 

 Hottonia pcdustns. 



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 ac- 

 quired, and must be the incidental result of differ- 

 ences in their vegetative systems. But how the 

 sexual elements of heterostyled plants came to differ 

 from what they were whilst the species was homo- 

 styled, and how they became co-adapted in two sets of 

 individuals, 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, 



