592 THE GENESIS OF NEW SPECIES. 



We shall have another opportunity of describing the way in which the lines of 

 demarcation of the ranges of entire floras become displaced in consequence of the 

 changes which the climate of a region is liable to undergo in course of time. These 

 displacements of floral regions are, as a rule, the result of very slow and inconspicu- 

 ous migrations on the part of the plants constituting the floras in question. The 

 direction of migration is invariably towards the places whose climatic conditions 

 agree best with the organization of the plants, and is, in the case of any one species, 

 either an advance or a retreat, according to the nature of the circumstances which 

 impel the species to migrate. The different plants of a flora do not all migrate in 

 a host together. Some species abandon their former home entirely and establish 

 themselves in a new locality more or less remote from it; others leave a few of 

 their kind behind in the old settlement at isolated spots which happen to be in 

 peculiarly favourable situations, and many succumb to the effects of the new con- 

 ditions or to the hardships incidental to the migration, and so die out. These 

 changes in the range of floras are naturally accompanied by all sorts of alterations 

 in the social relationships of the plants concerned especially with regard to the 

 co-existence of hybrids and their progenitors. It may happen that one or both 

 parent-species are left behind, whilst the hybrid advances, or the hybrid may 

 remain behind, whilst one of the parent-species advances; or, again, one of the 

 parent-stocks or both may die out. The facts concerning these local displacements 

 explain the phenomenon that species which, from their characteristics, may be 

 looked upon as hybrids of two other species, occupy in each case a district which 

 is separated, and often at a considerable distance, from the areas inhabited by the 

 species supposed to be their progenitors. The characteristics of the kind of Sorrel 

 named Rumex Patientia lead one to the conclusion that it is a hybrid derived from 

 Rumex aquaticus and Rumex erispus. It is found, however, growing wild in 

 Hungary and in Bosnia in parts where neither Rumex aquaticus nor R. Crispins 

 occurs at all. In Herzegovina there grows fairly commonly a Micromeria which 

 has been named by one of my friends Micromeria Kernen. So far as its charac- 

 teristics are concerned it must be considered to be a hybrid of Micromeria grceca 

 and Micromeria Juliana; yet neither of these two species grows in Herzegovina at 

 the present time, and they are not met with at any nearer spot than the part of 

 Dalmatia which stretches westward from Herzegovina, and belongs to the area of 

 distribution of the Mediterranean flora. In the little upland valleys of Planail and 

 Plawen, which run down from the mountains of the Oetzthal into the valley of the 

 Adige, there grows a Pulsatilla named Putsatilla nutans. If it occurred in company 

 with Pulsatilla vulgaris and Pulsatilla montana, all Botanists would be unanimous 

 in looking upon it as the product of a cross between those two species. Yet Pulsa- 

 tilla vulgaris and P. montana do not grow in the high valleys in question, but are 

 first met with at a distance of many miles from them, the former in the Unterinnthal 

 and the latter in the Vintschgau (a portion of the Adige valley). 



Inasmuch as the last-mentioned cases have to do with processes which have 

 taken place long ago they partly belong to the next chapter, where the genesis of 



