Unisexual Crosses 261 



crossing it with the parent-types. Such experi- 

 ments however, have miscarried owing to the 

 too stable nature of the unit-characters. 



This stability and this absence of the split- 

 ting shown by varietal marks in the offspring 

 of hybrids is one of the best proofs of unisex- 

 ual unions. It is often obscured by the accom- 

 panying varietal marks, or overlooked for this 

 reason. Only in rare cases it is to be met with 

 in a pure state and some examples are given of 

 this below. 



Before doing so, I must call your attention 

 to another feature of the unisexual unions. 

 This is the diminution of the fertilit}^, a phe- 

 nomenon universally known as occurring in 

 hybridizations. It has two phases. First, the 

 diminished chance of the crosses themselves of 

 giving full crops of seed, as compared with the 

 pure fertilization of either parent. And, sec- 

 ondly, the fertility of the hybrids themselves. 

 Seemingly, all grades of diminished fertility 

 occur and the oldest authors on hybrids have 

 pointed out that a very definite relation exists 

 between the differences of the parents and the 

 degree of sterility, both of the cross and of the 

 hybrid offspring. In a broad sense these two 

 factors are proportionate to each other, the 

 sterility being the greater, the lesser the affin- 

 ity between the parents. Many writers have 



