LIMITS TO THE EANGE OF PLAXT-SPECIES. 209 



down of a wood, may entirely alter the range of a species. Mr. 

 Dow has recorded the disappearance of Goody era repens^ R. Br., 

 from Longforgan, and Mr. Smith ^ from St. Fort Wood, through 

 this very simple cause. Drainage, railway and road-making 

 operations, all produce marked, definite, and distinct changes in 

 the flora. In this country the flora is wholly artificial over 

 nineteen-twentieths of the surface ; really wild plants are confined 

 to odd corners, banks, shingle-beds, &c. 



The best and clearest cases of naturally and definitely limited 

 range are found with certain marine plants. All true halophytes, 

 such as Suseda, Salsola, and Aster Trij^oluun, Linn., have in 

 course of time become adapted to salt-congested ground, and the 

 presence of salt is a sine quel non, in nature, of their existence. 

 If the salt be by any means removed, they are choked out by 

 others, just as the Mangrove is only able to flourish whilst the 

 sea water comes to its roots. When the soil rises above the level 

 of the tide, the Mangroves are choked out by the West Coast 

 Jungle.^ Yet, even in this respect, one must not be too certain. 

 I^ajas marina,^ Linn., was a fresh-water plant in the inter-glacial 

 period, it now lives in brackish or salt water ; and other plants 

 mav be alterin^r their habits. 



There are two other cases in which soil is admitted by most 

 people to distinctly limit specific range, viz., in plants confined 

 to peat or limestone countries. Of the two, peat, in my own 

 experience, exercises the more rigorous selection. Yet almost 

 all plants which are found on peat would grow in other places, 

 if they were allowed to do so by the struggle for existence. Such 

 a species as Potentilla Tormentilla, Scop,, is extremely common, 

 even dominant, on peat, but it also grows perfectly well on other 

 soils. Limestone and chalk plants are not so distinctly limited, 

 for it is only on bare exposed knolls and rocks that the character- 

 istic lime-loving species are found, or, at any rate, only where the 

 limestone can really afiect the plant. There are, however, a few 

 plants to whose distribution peat or limestone is of the nature 

 of a sine qud non. 



^ Smith, Proc. Perthshire Soe. Nai. Science, Vol. II., Part vi., 1S9S; 



- Colonial Reports, Miscellaneous, No. 3, p. 6. 



3 Krause, Botan. Centralbkitt, Bd. LXXV., No. 3, p. 66. 



