INSTANCES OF SUCCESSFUL HYBRIDS. 589 



crossed with pollen of their own kind. If no insects appear upon the scene, auto- 

 gamy takes place. In either case fertile seeds are developed, and give rise to plants 

 which do not differ from the parent-form. In places where detritus from the 

 adjacent limestone and dolomitic cliffs is mixed with the humus of the soil the 

 seedlings thrive distinctly better than those of R. ferrugineum and no less well 

 than those of R hirsutum. Thus, so far as the soil is concerned, R intermedium 

 has an advantage over R. ferrugineum, and in respect of insect-visits it is better 

 off than either of the parent-species. These advantages, though apparently insig- 

 nificant, are not merely sufficient to prevent R intermedium from yielding to the 

 parent-species in the struggle for existence at the places in question, but even give 

 it rather a better chance of prevailing. 



As a second instance, we will take Salvia sylvestris, the hybrid offspring of 

 Salvia nemorosa and Salvia pratensis. This plant grows in dry meadows all over 

 the low country to the south of Vienna, as, for instance, on the banks of the river 

 Leitha, which separates Austria proper from Hungary. The landscape there is 

 slightly undulating, the elevations are composed of boulders and clay, and wherever 

 the latter is present in great quantities, especially on the gentle slopes of the rising 

 ground, Salvia nemorosa constitutes an important item in the vegetation. The 

 hollows are full of a dark moist earth, with a rich admixture of humus, and there 

 we find meadows in which Salvia pratensis grows luxuriantly. These two kinds 

 of habitat usually pass quite gradually into one another, and the parts common to 

 both consist of dry meadow-lands. Salvia nemorosa does not thrive in the intri- 

 cate grass-carpet of these meadows, and is rarely to be seen there, whilst for Salvia 

 pratensis the soil is too dry, so that that species also is but poorly represented on 

 the areas in question. On the other hand, these dry meadows are the most suitable 

 ground for the hybrid Salvia sylvestris, and it thrives exceedingly upon them. 

 Its flowers are much visited by insects; its fruits ripen in as large numbers as 

 in the case of S. nemorosa or S. pratensis, and have been found by experiment to 

 be fertile in a proportion of more than 60 per cent. Salvia sylvestris has 

 therefore scattered itself all over this dry meadow-land, and manifests all the 

 characteristics essential to our conception of a species. 



A third example is Nuphar intermedium, a hybrid derived from Nuphar 

 luteum and Nuphar pumilum. This plant grows in lakes in the Black Forest 

 and in the Vosges. It is also scattered over North Germany, and occurs with 

 increasing frequency in Central and Northern Russia and in Sweden. It has been 

 found as far north as Lapland. At the northern extremity of this large area of 

 distribution Nuphar intermedium is more abundant than the species from which 

 it is derived; indeed in many places it occurs in their absence, and in fact passes 

 beyond the northern limits of their area of distribution. In these situations there 

 is, of course, no possibility of the hybrid's crossing with either of the parent- 

 species or of the formation of goneoclinic hybrids. Nuphar intermedium subsists 

 independently there, multiplies without change of form, and has in fact established 

 itself as a species. This phenomenon is explained as follows: The northern limit of 



