I 



1881-82.] Edinburgh Naturalists' Field Club. 15 



observed from the tropics to the arctics." Nature affords several 

 effective means for the distribution of seeds, — either by currents of 

 air or water, birds, or fiirred animals ; so that if elevation should go 

 on in the tropics, the means are at hand to furnish a flora which 

 shall succeed a tropical flora that cannot ascend beyond a certain 

 limit. To this sub-tropical flora would succeed one characteristic 

 of temperate regions, should circumstances permit. Depression 

 going on to a sufficient extent would produce in a country possess- 

 ing different degrees of elevation a series of islands ; so that if we 

 find a series of islands possessing a similar flora, it is reasonable to 

 think that they may at some previous time have been united. A 

 converse process of upheaval going on in an archipelago would 

 convert a number of islands into a continuous belt of land, so that 

 in this case the conditions are favourable for the appearance of a 

 uniform flora. 



We find at the present day that the means which nature employs 

 for scattering seeds over a large extent of the earth's surface pro- 

 duce effective results. A high wind will scatter the seeds of plants 

 that have lived on hilly ground over the marsh, meadow, and sea- 

 shore. The seeds of maritime plants must often be carried far 

 inland ; while it is needless to speak of the effects which sea-cur- 

 rents produce in conveying seeds from one part of the globe to the 

 other. Thus there is every chance given for an intermixture of 

 plants belonging to different habitats. But in reality, do we, as a 

 general rule, find the marsh plant growing alongside the agrarian 

 weed, or the littoral plant with that of the wood ? Speaking broadly, 

 we do not. If, then, the plants characteristic of lowland districts 

 will not grow in the sea-marsh, what will happen should a tract of 

 level country be turned into a sea-swamp ? If the change be 

 sudden, then there is no doubt but that maritime plants will 

 speedily exterminate all the others. But if the change be very 

 gradual, then there is great reason to think that some, if not 

 aU, of the plants which grew on the open level ground may 

 gradually adapt themselves to the new conditions of life. In this 

 way varieties may arise. Some plants would seem to be able to 

 defy extermination. There ai'e several cosmopolitan genera which 

 no change of climate would cause to become extinct. Polygonum 

 amphibium grows in ponds and on dry land. Some plants in this 

 country range from the sea- shore to alpine limits, while others 

 would seem to take a leap from the sea-shore to the mountain, or 

 vice versa. It follows, then, that should the marsh be suddenly 

 elevated and drained, those plants which can grow both on moist 

 ground and on dry would not become extinct. Again, the mari- 

 time plants which in this country are found at alpine limits would 

 still continue to exist, even though the sea-line were rapidly ele- 

 vated. To account for the fact of Cochlearia officinalis, Armeria 



