260 HYBRIDISM. 



CHAPTER IX. 



HYBRIDISM. 



Distinction between the Sterility of First Crosses and of Hybrids — 

 Sterility Various in Degree, not Universal, affected by Close Inter- 

 breeding, removed by Domestication — Laws governing the Steril- 

 ity of Hybrids — Sterility not a Special Endowment, but Incidental 

 on Other Differences, not accumulated by Natural Selection — 

 Causes of the Sterility of First Crosses and of Hybrids — Parallel- 

 ism between the Effects of Changed Conditions of Life and of 

 Crossing — Dimorphism and Trimorphism — Fertility of Varieties 

 when crossed and of their Mongrel Offspring not Universal — 

 Hybrids and Mongrels compared independently of their Fertility 

 — Summary. 



The view commonly entertained by naturalists is that 

 species, when intercrossed, have been specially endowed 

 with sterility, in order to prevent their confusion. This 

 view certainly seems at first highly probable, for species 

 living together could hardly have been kept distinct had 

 they been capable of freely crossing. The subject is in 

 many ways important for us, more especially as the sterility 

 of species when first crossed, and that of their hybrid off- 

 spring, cannot have been acquired, as I shall show, by the 

 preservation of successive profitable degrees of sterility. 

 It is an incidental result of differences in the reproductive 

 systems of the parent-species. 



In treating this subject, two classes of facts, to a large 

 extent fundamentally different, have generally been con- 

 founded ; namely, the sterility of species when first crossed, 

 and the sterility of the hybrids produced from them. 



Pure species have of course their organs of reproduction 

 in a perfect condition, yet when intercrossed they produce 

 either few or no offspring. Hybrids, on the other hand, 

 have their reproductive organs functionally impotent, as 

 may be clearly seen in the state of the male element in 

 both plants and animals ; though the formative organs 

 themselves are perfect in structure, as far as the micro- 

 scope reveals. In the first case the two sexual elements 

 which go to form the embryo are perfect ; in the second 



