Chap. VIII. LAWS OF STERILITY. 255 



circumstances 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 together in utter 

 confusion. The following rules and conclusions are 

 chiefly drawn up from Gartner's admirable work on the 

 hybridisation of plants. I have taken much pains to 

 ascertain how far the rules apply to animals, and con- 

 sidering how scanty our knowledge is in regard to hy- 

 brid animals, I have been surprised to find how gene- 

 rally the same rules apply to both kingdoms. 



It has been already remarked, that the degree of fer- 

 tility, both of first crosses and of hybrids, graduates 

 from zero to perfect fertility. It is surprising in how 

 many curious ways this gradation can be shown to 

 exist ; but only the barest outline of the facts can here 

 be given. When pollen from a plant 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 differ- 

 ent species of the same genus applied to the stigma of 

 some one species, yields a perfect gradation in the 

 number of seeds produced, up to nearly complete or 

 even quite complete fertility; and, as we have seen, 

 in certain abnormal cases, even to an excess of fertility, 

 beyond that which the plant's own pollen will produce. 

 So in hybrids themselves, there are some which never 

 have produced, and probably never would produce, even 

 with the pollen of either pure parent, a single fertile seed : 

 but in some of these cases a first trace of fertility may 

 be detected, by the pollen of one of the pure parent- 

 species 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 to be a sign 



