Chapter VIH —123— Natural Factors 



mountain species and only a very few subalpine species and these such 

 as are found also at lower elevations, while there are a considerable 

 number of species that are found at all elevations and, hence, are not 

 capable of furnishing any data that might give a clue to their origin. 

 Consequently, Amann came to the conclusion that the mosses found on 

 erratic boulders in Switzerland cannot serve as proof of the transport 

 by glaciers of the seeds of alpine plants from mountains to plains 

 during the Ice Age. It is much more probable that they invaded the 

 plains later during the course of the present geological period. 



Pettersson (1929) investigated the plants growing on erratic blocks 

 (boulders) near Helsingfors, and he came to the conclusion that these 

 plants belong to the surrounding flora. Seeds from this flora, borne 

 thither by wind or other agents, germinated on these huge boulders 

 after the latter had arrived at their present location. 



The foregoing suffices to illustrate the undoubted significance of 

 water, in its different manifestations, as a factor in the dispersal of 

 plants (see also Gtjppy, 1917) along sea and lake shores and river 

 banks, but it does not provide an adequate basis for establishing the 

 possibility of the transport of plants for any great distance inland. 

 Even in those cases when seeds are borne by sea currents for great 

 distances, this rarely has any significance in the founding of new, 

 isolated habitats of these plants. 



AxPHONSE DE Candolle corrcctly emphasized the fact that, though 

 rivers may bear the seeds and fruits of plants from the Alps into the 

 plains or for considerable distances from north to south or from south 

 to north, they often bring them to regions where they cannot establish 

 themselves. This holds true also for sea currents, which may carry 

 seeds for immense distances. But even if such seeds, despite prolonged 

 immersion in sea water, preserve their germinating power, they play, 

 from a phytogeographical point of view, a very insignificant r61e. 

 Sea currents usually do not flow parallel to the equator, and so often 

 the seeds are borne into regions where the climatic conditions are 

 entirely alien to them, not to mention the fact that one or two seeds 

 are in most cases quite inadequate for a plant to become naturalized 

 and establish itself as a component part of an already existing vege- 

 tation. 



Animals as a Factor in the Dispersal of Plants: — That animals 

 play an important role in the dispersal of plants is beyond doubt. 

 Nevertheless, it is clear that in most cases seeds are borne by animals 

 over comparatively short distances. For an elucidation of the question 

 interesting us — the possibility of explaining discontinuous areas of 

 species by the chance transport of seeds — only a few kinds of animals, 

 chiefly birds and, to some extent, fish (that carry seeds of water 

 plants), are of any significance. Hence, we shall consider primarily the 

 role of birds in the dispersal of plants. The significant role that birds 

 may play in the dispersal of plants has been pointed out time and again, 

 beginning with Theophrastus, who noted that the mistletoe {Viscum 

 album) is dispersed by birds. 



Birds may serve in two ways for the distribution of seeds over great 

 distances: (/) evacuation of undigested seeds with their excreta 



