Unisexual Crosses 267 



standing between two allied types, may be con- 

 sidered to have originated by a cross. Surely 

 a wide field for abuse is opened by such an as- 

 sertion, and it is now a common habit to con- 

 sider intermediate forms as hybrids, on the 

 grounds afforded by their external characters 

 alone, and without any exact knowledge of their 

 real origin and often without knowing anything 

 as to their constancy from seed. All such ap- 

 parent explanations are now slowly becoming 

 antiquated and obsolete, but the cases adduced 

 by Kerner seem to stand this test. 



Kerner designates a willow, Salix Ehrhart- 

 iana as a constant hybrid between Salix alba 

 and S. pentandra. Rhododendron intermed- 

 ium is an intermediate form between the hairy 

 and the rusty species from the Swiss Alps, R. 

 hirsutum and R. ferrugineum, the former grow- 

 ing on chalky, and the other on silicious soils. 

 Wherever both these types of soil occur in the 

 same valley and these two species approach 

 one another, the hybrid R. intermedium is pro- 

 duced, and is often seen to be propagating itself 

 abundantly. As is indicated by the name, it 

 combines the essential characters of both par- 

 ents. 



Linaria italic a is a hybrid toad-flax between 

 L. genistifolia and L. vulgaris, a cross which I 

 have repeated in my garden. Drosera obovata 



