Bisexual Crosses 279 



in unisexual unions in all possible degrees, ac- 

 cording to the amount of difference between the 

 parents. If this amount is slight, if for in- 

 stance, only one unit-character causes the dif- 

 ference, the injury to fertility may be so small 

 as to be practically nothing. Hence we see that 

 this test would not enable us to judge of the 

 doubtful cases, although it is quite sufficient as 

 a proof in cases of wider differences. 



Our second assertion related to the reciprocal 

 crosses. This is the name given to two sexual 

 combinations between the same parents, but 

 with interchanged places as to which furnishes 

 the pollen. In unisexual crosses of the genus 

 Oenothera the hybrids of such reciprocal unions 

 are often different, as we have formerly seen. 

 Sometimes both resemble the pollen-parent 

 more, in other instances the pistil-parent. In 

 varietal crosses no such divergence is as yet 

 known. It would be quite superfluous to ad- 

 duce single cases as proofs for this rule, which 

 was formerly conceived to hold good for hy- 

 brids in general. The work of the older hybrid- 

 ists, such as Koelreuter and Gaertner affords 

 numerous instances. 



Our third rule is of a wholly different nature. 

 Formerly the distinction between elementary 

 species and varieties was not insisted upon, and 

 the principle which stamps retrograde changes 



