240 LAWS GOVERNING THE STERILITY Chap. VIII. 



species of animals when crossed ; or we must look at sterility, 

 not as an indelible characteristic, but as one capable of being 

 removed by domestication. 



Finally, considering^ all the ascertained facts on the inter- 

 crossing of plants and animals, it may be concluded that some 

 degree of sterility, both in first crosses and in hybrids, is an 

 extremely general result ; but that it cannot, under our present 

 state of knowledge, be considered as absolutely univ'crsal. 



Laics f/overnlng the Sterility of First Grosses and of Hybrids. 



We will now consider a little more in detail the circum- 

 stances and rules governing the sterility of first crosses and of 

 hybrids. Our chief object will be to see whether or not the 

 rules indicate that species have specially been endowed with 

 this quality, in order to prevent their crossing and blending to- 

 gether in utter confusion. The following rules and conclusions 

 are chiefly drawn \\\) from Gartner's admirable work on the 

 hybridization of plants. I have taken much pains to ascertain 

 how far the rules apply to animals, and, considering how scanty 

 our knowledge is in regard to hybrid animals, I have been sur- 

 prised to find how generally the same rules apply to both king- 

 doms. 



It has been already remarked, that the degree of fertilitj'', 

 both of first crosses and of hybrids, graduates from zero to per- 

 fect fertility. It is surprising in how many curious ways this 

 gradation can be shown ; but only the barest outline of the 

 facts can here be given. When pollen from a ])lant of one 

 family is placed on the stigma of a plant of a distinct family, it 

 exerts no more influence than so much inorganic dust. From 

 this absolute zero of fertility, the pollen of dilferent species of 

 the same genus applied to the stigma of some one of the 

 species, 3-ields a jierfect gradation in the number of seeds pro- 

 duced, up to nearly comj)lete or even quite comi)lete fertility ; 

 and, as we have seen, in certain abnormal cases, even to an 

 excess of fertility, beyond tiiat which the plant's own pollen 

 j^roduces. So in hybrids themselves, there are some which 

 never have produced, and probably never would produce, even 

 with the pollen of the pure parents, a single fertile seed : but 

 in some of these cases a first trace of fertility mvcy be detected, 

 by the jiollen of one of the pure parent-sjiecies causing the 

 flower of the hybrid to wither earlier than it otherwise would 

 have done ; and the early withering of the flower is well known 



