THE DISTRIBUTION OF HYBRIDS. 591 



lower limits of the Alpine Flora. Another characteristic of these borderlands is 

 the fact that the separate hybrids growing on them are almost always represented 

 by a large number of individuals, and the consequence is that there is no great 

 chance of their crossing with the parent-species and gradually reverting, in succes- 

 sive generations, to those species. If isolated individuals belonging to a particular 

 hybrid grow in the midst of thousands of plants of the parent-stocks, their stigmas 

 will, in all probability, be dusted with pollen from the latter species. This proba- 

 bility diminishes, however, as the number of individuals of the parent -species 

 flowering in the same locality as the hybrid diminishes; if that number is small 

 the hybrid is thrown mainly upon its own resources for fertilization, and, provided 

 it is self-fertile, there is nothing to prevent its multiplying and becoming dispersed. 



Connected with the above is the further fact that in the neighbourhood of a 

 hybrid which has become a species there is sometimes no trace of one of the parent- 

 species, it having completely died out. The slightest change in climatic conditions 

 may cause the plants of a particular stock to succumb at the confines of the stock's 

 area of distribution, where they are only present in small numbers, and are anything 

 but strong; and when this happens we find the other parent-species alone growing 

 side by side with the hybrid, and even that species is possibly not so well adapted 

 as the hybrid to the altered conditions. Of cases in point we will select two from 

 the East of Europe and two from the West. 



When Epilobium alsinefolium and Epilobium palustre are crossed a hybrid is 

 obtained which, when fertilized with its own pollen, yields a large quantity of fertile 

 seeds. The plants reared from these seeds exhibit the same characteristics as 

 the plant from which the seeds were taken. This hybrid grows together with 

 the parent-species in the Biesengebirge, and has received the name of Epilobium 

 scaturiginum. It is likewise a native of the Bihar Gebirge, on the confines of 

 Hungary and Transylvania, and is of very common occurrence in the springs and 

 rivulets in the vicinity of the Hochkamm (a mountain of this chain). Yet, of the 

 two parent-species, only one, viz. Epilobium palustre, grows amongst these moun- 

 tains. Again, Prunella hybrida is a hybrid springing from Prunella laciniata 

 and Prunella vulgaris. It is widely distributed in the Wienerwald district, and in 

 some places is commoner than the parent-species, whilst in Moravia and Bohemia it 

 occurs in places where one of the latter, viz. Prunella laciniata, is entirely absent. 

 A third instance is afforded by Primula brevistyla, called also Primula variabilis, 

 a hybrid derived from Primula vulgaris and P. officinalis. This plant is true to 

 seed, and is met with everywhere throughout almost the whole of Europe in 

 company with both parent-species. In some districts of France it is found also in 

 places where one or other of the latter species does not grow at all, and even where 

 both are absent. Our fourth example, Linaria stricta, is the hybrid of Linaria 

 striata and Linaria vulgaris. It occurs in many places in the West of Europe, 

 together with its progenitors, but in the South of France, in the neighbourhood of 

 Montpellier, it is found growing with Linaria striata alone, whilst the other parent- 

 stock, L. vulgaris, is never found in the district. 



