Chap. VIII. DIMORPHISM AND TRIMOKPIllSM. 253 



on, or consists in, the incessant action and reaction of various 

 forces, which, as throui^liout Nature, are always tending toward 

 an e(iuilihriuiii ; and when this tendency is slip^htly disturbed 

 by any change, tlie vital forces apparently gain in power. 



Meciprocal Dimorphism and Trimorpliism. 



This subject may he here briefly discussed, and will l)e 

 found to throw some light on hybridism. Several plants belong- 

 ing to distinct orders present two forms, which exist in al)out 

 equal numbers, and whicli differ in no respect except in tlieir 

 re])roductive organs ; one form having a long pistil with short 

 stamens, tlie other a short ])istil with long stamens; botli with 

 dilVerently-sized pollen-grains. With trimorphic plants there 

 are three forms likewise difl'ering in the lengths of their pistils 

 and stamens, in the size and color of the pollen-grains, and in 

 some otlier respects ; and as in each of the three forms there 

 are two sets of stamens, there arc altogether six sets of stamens, 

 and three kinds of pistils. These organs are so proportioned 

 in length to each other, that, in any two of the forms, half the 

 stamens in each stand on a level with the stigma of the third 

 fonn. Now I have shown, and the result has been confirmed 

 Ijy other observers, that, in order to obtain full fertility Avith 

 these ])lants, it is necessary that the stigma of the one form 

 should be fertilized by pollen taken from the stamens of corre- 

 sponding height in the other form. So that with dimorphic 

 species two unions, wliich may be called legitimate, arc fully 

 fertile ; and two, which may be called illegitimate, are more 

 or less infertile. AVith tnmorj>hic species six imions are legiti- 

 mate or fully fertile, and twelve are illegitimate or more or 

 less infertile. 



The infertility which may be observed in various dimorphic 

 and trimorphic ])lants, when they are illegitimately fertilized, 

 that is, by ]io]Um) taken from stamens not coi'responding in 

 height villi the pistil, dilTers much in degree, up to absolute 

 and utter sttM-ility ; just in the same manner as occurs in cross- 

 ing distinct species. As the degree of sterility in the latter 

 case depends in an eminent degree on the conditions of life 

 being more or less favorable, so I have found it witli iUegitimate 

 imions. It is well known that if pollen of a distinct species 

 be placed on the stigma of a flower, and its own ]iollen be 

 afterward, even after a considerable interval of time, placed on 

 the same stigma, its action is so strongly prepotent that it 



