Chap. XIX. HYBRIDISM. 165 



excej)tion of fertility, the most striking accordance in all other 

 respects ; namely, in the laws of their resemblance to their 

 two parents, in their tendency to reversion, in their varia- 

 bility, and in being absorbed through repeated crosses by 

 either parent- form. 



After arriving at these conclusions, T was led to investigate 

 a subject which throws considerable light on hybridism, 

 namely, the fertility of heterostyled or dimorphic and 

 triinorphic plants, when illegitimately united. I have had 

 occasion several times to allude to these plants, and I may 

 here give a brief abstract of my observations. Several plants 

 belonging to distinct orders present two forms, which exist 

 in about equal numbers, and which differ in no respect except 

 in their reproductive organs ; one form having a long pistil 

 with short stamens, the other a short pistil with long 

 stamens ; both with differently sized pollen-grains. With 

 trimorphic plants there are three forms likewise differing in 

 the lengths of their pistils and stamens, in the size and colour 

 of the pollen-grains, and in some other respects ; and as in 

 each of the three forms there are two sets of stamens, there 

 are altogether six sets of stamens and three kinds of pistils. 

 These organs are so proportioned in length to one another 

 that, in any two of the forms, half the stamens in each stand 

 on a level with the stigma of the third form. Kow I have 

 shown, and the result has been confirmed by other observers, 

 that, in order to obtain full fertility with these plants, it is 

 necessary that the stigma of the one form should be fertilised 

 by pollen taken from the stamens of corresponding height in 

 the other form. So that with dimorphic species two unions, 

 which may be called legitimate, are fully fertile, and two, 

 which may be called illegitimate, are more or less infertile. 

 With trimorphic species six unions are legitimate, or fully 

 fertile, and twelve are illegitimate, or more or less infertile.^ 



The infertility which may be observed in various dimorphic 



* My observations 'On the Cha- Linnean Soc.,' vol. x. p. 393. The 



racter and hybrid-like nature of the abstract here given is nearly the 



offspring from the illegitimate union same with that which appeared in 



of Dimorphic and Trimorphic Plants ' the 6th edition of my ' Origin of 



urere published in the ' Journal of the Species.* 



