OF FIRST CROSSES AND OF HYBRIDS. 279 



subjected, 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 parallelism 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 essen- 

 tially related to the principle of life ; this principle, accord- 

 ing 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 

 toward an equilibrium ; 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 likewise differing in 

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

 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 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 



