Chap. IX.] DIMORPHISM AND TEIMORPHISM. 29 



principle of life ; this principle, according to Mr. Herbert 

 Spencer, being that life depends on, or consists in, the 

 incessant action and reaction of various forces, which, 

 as throughout nature, are always tending towards an 

 equilibrium ; and when this tendency is slightly dis- 

 turbed by any change, the vital forces gain in power. 



Reciprocal Dimorphism and Trimorphism. 



This subject may be here briefly discussed, and will 

 be found to throw some light on hybridism. 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 ; the two having 

 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, 

 the three forms possess altogether six sets of stamens 

 and three kinds of pistils. These organs are so pro- 

 portioned in length to each other, that half the stamens 

 in two of the forms stand on a level with the stigma of 

 the third form. Now 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 another 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 



