PLANT-MIGRATIOXS. 883 



particularly those with south-east and south-west aspects, are much greater than is 

 commonly supposed, and species are known, for example, which in a particular 

 district invariably have their habitats on south-east slopes, whilst others occur only 

 on slopes facing south-west. 



It has been stated that the geological phenomena, and the changes of climate 

 connected therewith, have at all epochs exercised an important influence on the 

 migrations of plants, and have consequently had much to do with the displacements 

 that have occurred in the lines of distribution. If a change occurs which results in 

 the climatic conditions of 48 north latitude becoming such as previously prevailed 

 at 46 north latitude, those species of plants whose range extended northwards as 

 far as 46 proceed to take possession of suitable habitats beyond that limit and the 

 northern line of vegetation of those particular species is sooner or later shifted north- 

 wards. On the other hand, the species which had up to that time enjoyed favourable 

 conditions at 48 north latitude, but can no longer nourish under the new conditions, 

 establish themselves on more suitable habitats lying further north, so that their 

 southern line of vegetation undergoes displacement northwards. The eastern and 

 western lines of vegetation may also be shifted in accordance with such alterations 

 of climate as occur when an inland locality is converted into a maritime site or vice 

 versa. These displacements may assume the aspect of a progression or of a retro- 

 gression, but in every instance the abandonment of the area of settlement will 

 take the form of a migration of the plant-species concerned. 



These migrations of plants which are accomplished independently of human 

 influence take place as slowly or as quickly as the changes of climate to which they 

 are due. In the case of species only capable of flourishing on particular habitats 

 colonization must naturally proceed by leaps and bounds, whilst retrogression also 

 cannot possibly take place uniformly. 



The numerous habitats occupied by a species within the limits of its area of 

 distribution are scarcely ever exactly alike in respect of the conditions which 

 influence plant-life. Some habitats are pre-eminent on account of their advantageous 

 position. The species in question develops most robustly, and multiplies most 

 abundantly in that habitat. In the event, too, of a change of climate the species 

 continues to live there longer than elsewhere, and may succeed in clinging to such 

 isolated spots under fundamentally altered climatic conditions long after it has died 

 out from hundreds of other habitats in the neighbourhood. If the species has 

 meanwhile transplanted itself to adjoining territory and established there a fresh 

 area of distribution, the spots where it has managed to survive in the old country 

 appear like lost outposts wrested from the main area of distribution, or like islands 

 lying off the shores of a continent. Such a state of things is by no means uncommon, 

 and we are able to deduce therefrom facts not only concerning the former condition 

 of the vegetable world, but also concerning the climatic conditions which used to 

 prevail and as to the directions in which, in due course, plant-migrations have taken 

 place. We shall have an opportunity to return to these interesting cases in the last 

 chapter of this volume. 



