586 THE GENESIS OF NEW SPECIES. 



the lower Danube ; Roripa anceps, the hybrid of Roripa amphibia and R. sylvestris, 

 is met with all over the district forming the basin of the Baltic; Primula digenea, 

 a hybrid derived from Primula vulgaris and P. elatior, grows in its thousands in 

 the upland meadows of the Eastern Alps; Betula alpestris, produced by the crossing 

 of Betula alba and B. nana, is met with in great abundance in the Jura, in Scandi- 

 navia, and in the North of Russia, and here and there entire copses are composed 

 of it; Nigritella suaveolens, a hybrid produced from Gymnadenia conopsea and 

 Nigritella nigra, is so common in the Central Alps, for instance, in the Pusterthal 

 of Tyrol, that hundreds of specimens are sometimes encountered in a single meadow; 

 Primula Salisbiirgensis, the hybrid of Primula glutinosa and P. minima, is repre- 

 sented by a host of plants on the Tyrolese Alps, as, for instance, on the Muttenjoch 

 and the neighbouring mountains which separate the Gschnitzthal from the Obern- 

 bergthal. 



Several hundreds of other cases of the kind might be mentioned, but the limits 

 of this book will not allow me to do more than give the following names selected 

 from the long list that lies before me : 



HYBRID. PARENTS. 



Asplenium Germanicum Asplenium Ruta-muraria x A. septentrionale. 



Calamogrostis acutiflora C. arundinacea x C. epigeios. 



Carex Bcenninghausiana C. paniculata x C.remota. 



Scirpus Duvalii S. lacustris x S. Pollichii. 



Juncus diffusus J. effusus x J. glaucus. 



Orchis Dietrichiana 0. tridentata x 0. ustulata. 



Potamogeton spathulatus P. polygonifolius x P. rufescens. 



Populus canescens P. alba x P. tremula. 



Salix Austriaca S. grandifolia x S. purpurea. 



Rumex maximus R. aguaticus x R. Hydrolapathum 



Polygonwn mile. P. Hydropiper x P. Persicaria. 



Cistus Florentinus G. monspeliensis x C. salvifolius. 



Draba Hoppeana D. Fladnizensis x D. Carinthiaca. 



Roripa stenocarpa R. palustris x R. sylvestris. 



Pulsatilla Hakelii P. patens x P. pratensis. 



Drosera obovata D. longifolia x D. rotundifolia. 



Epilobium salidfolium E. alsinifolium x E. montanum. 



Sorbus latifolia S. Aria x S. torminalis. 



Potentilla procumbens P. erecta x P. reptans. 



Mentha nemorosa M. aquatica x M . sylvestris. 



Pedicularis atrorubens P. incarnata x P. recutita. 



Verbascum rubiginosum V. Austriacum x V. phoeniceum. 



A canthus spinulosus A . mollis x A . spinosissimus. 



Gentiana Charpentieri G.lutea x G. punctata. 



Primula pubescens P. Auricula x P. hirsuta. 



Vaccinium intermedium V. Myrtillus x V. Vitis-idaea. 



Erica Mackayi E. ciliaris x E. Tetralix. 



Cirsium tataricum C. canum x C. oleraceum. 



Lappa pubens . L. minor x L. tomentosa. 



Hieraciurn stolonijlorum H. aurantiacum x H. pilosellceforme. 



The fact that hybrids exhibit every conceivable degree of frequency of occur- 

 rence might lead one to suppose that the rare hybrids were those that had been most 

 recently produced, and that they exist only as isolated plants because they have not 



