Unisexual Crosses 269 



p r i m us." The latter is a cross between the 

 California!! dewberry and the Siberian rasp- 

 berry and is certainly to be regarded as a good 

 stable species, artificially produced. Bell 

 Salter crossed the willow-herbs Epilobium tet- 

 ragonum and E. montanum, and secured inter- 

 mediate hybrids which remained true to their 

 type during four successive generations. 



Other instances might be given. Many of 

 them are to be found in horticultural journals 

 and in the botanical writings which treat of 

 systematic and anatomical details. The ques- 

 tion of stability is generally dealt with in an 

 incidental manner, and in many cases it is diffi- 

 cult to reach conclusions from the facts given. 

 Especially disturbing is the circumstance that 

 from a horticultural point of view it is quite 

 sufficient that a new type should repeat itself in 

 some of its offspring, to call it stable, and that 

 for this reason the absolute constancy is rarely 

 proved. 



The range of constant hybrids would be 

 larger by far were it not for two facts. The 

 first is the absolute sterility of so many beauti- 

 ful hybrids, and the second is the common occur- 

 rence of retrogressive characters among culti- 

 vated plants. To describe the importance of 

 both these groups of facts would take too much 



