260 CONCLUDING EEMAEKS Chap. Vlt 



large number of seeds, and this obviously is a very 

 common case, beterostyled will have an advantage 

 over dioecious plants, as all the individuals of the 

 former, whilst only haK of the latter, that is the 

 females, yield seeds. On the other hand, hetero- 

 styled plants seem to have no advantage, as far as 

 cross-fertilisation is concerned, over those which are 

 sterile with their own pollen. They lie indeed under 

 a slight disadvantage, for if two self-sterile plants 

 grow near together and far removed from all other 

 plants of the same species, they will mutually and 

 perfectly fertilise one another, whilst this will not be 

 the case with heterostyled dimorphic plants, unless 

 they chance to belong to opposite forms. 



It may be added that species which are trimorphic 

 have one slight advantage over the dimorphic ; for if 

 only two individuals of a dimorphic species happen 

 to grow near together in an isolated spot, the chances 

 are even that both will belong to the same form, and 

 in this case they will not produce the full number of 

 vigorous and fertile seedlings ; all these, moreover, 

 wiU tend strongly to belong to the same form as their 

 parents. On the other hand, if two plants of the same 

 trimorphic species happen to grow in an isolated spot, 

 the chances are two to one in favour of their not be- 

 longing to the same form ; and in this case they will 

 legitimately fertilise one another, and yield the full 

 complement of vigorous offspring. 



The Means by which Plants may have leen rendered 

 Heterostyled, 



This is a very obscure subject, on which I can throw 

 little light, but which is worthy of discussion. It has 



