252 RECIPROCAL DIMORPHISM [Chap. IX. 



Hence it seems that, on the one hand, slight changes in the con- 

 ditions of life benefit all organic beings, and on the other hand, that 

 slight crosses, that is crosses between the males and females of the 

 same species, which have been subjected to slightly different con- 

 ditions, or which have slightly varied, give vigour and fertility to 

 the offspring. But, as we have seen, organic beings long habituated 

 to certain uniform conditions under a state of nature, Avhen sub- 

 jected, as under confinement, to a considerable change in their 

 conditions, very frequently are rendered more or less sterile ; and 

 we know that a cross between two forms, that have become widely 

 or specifically different, produce hybrids which are almost always in 

 some degree sterile. I am fully persuaded that this double paral- 

 lelism is by no means an accident or an illusion. He who is able 

 to explain why the elephant and a multitude of other animals are 

 incapable of breeding when kept under only partial confinement in 

 their native country, will be able to explain the primary cause of 

 hybrids being so generally sterile. He will at the same time be 

 able to explain how it is 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 together 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 throughout nature, are always tending towards an equi- 

 librium ; and when this tendency is slightly disturbed 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 like- 

 wise 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 proportioned in length to each other, 



