CHAPTER IX 

 Hybridism 



Distinction between the Sterility of First Crosses and of Hybrids — SteriOlity 

 Various in Degree, not Universal, affected by Close Interbreeding, re- 

 moved by Domestication — Laws governing the Sterility 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 — ^Parallelism 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 

 Fertihty — 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 sub- 

 ject is in many ways important for us, more especially as the ster- 

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

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

 vation of successive profitable degrees of sterility. It is an inci- 

 dental 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 confounded; 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 reproduc- 

 tive 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 

 microscope reveals. In the first case the two sexual elements which 

 go to form the embryo are perfect; in the second case they are 

 either not at all developed, or are imperfectly developed. This dis- 



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