236 RECIPROCAL DIMORPHISM [CHAP. IX 



that the races of some of our domesticated animals, which have 

 often been subjected to new and not uniform conditions, are quite 

 fertile together, although they are descended from distinct species, 

 which would probably have been sterile if aboriginally crossed. 

 The above two parallel series of facts seem to be connected to- 

 gether by some common but unknown bond, which is essentially 

 related to the 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 through- 

 out nature, are always tending towards an equilibrium ; and when 

 this tendency is slightly disturbed by any change, the vital forces 

 gain in power. 



Reciprocal Dimorphism and Trlmorpkism. 



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 repro- 

 ductive 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 legitimate, or fully fertile, and twelve are 

 illegitimate, or more or less infertile. 



The infertility which may be observed in various dimorphic 

 and trimorphic plants, when they are illegitimately fertilised, that 

 is by pollen taken from stamens not corresponding in height with 

 the pistil, differs much in degree, up to absolute and utter 

 sterility ; just in the same manner as occurs in crossing 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 

 favourable, so I have found it with illegitimate unions. It is 

 well known that if pollen of a distinct species be placed on th& 



