LIMITS TO THE RANGE OF PLANT-SPECIES. 213 



limiting factors for fresh- water forms are, first of all, the existence 

 or lochs or running water, and then the depths, character of the 

 bottom, isothermal lines, and altitudes. 



If we subtract all these groups from the British Flora, we have 

 left the natural wild plants of the woods, Bramble clumps. Broom 

 and Whin lands, river-sides, and so on. If we consider the changes 

 the country has passed through, even in the historic period, the 

 destruction of the Caledonian Forest, replanting, the introduction 

 of Larch woods, the abandoning of Larch for other trees, the 

 planting of sand-dunes and peat-mosses, and so on, it is obvious 

 that it is, in the highest degree, rash to say that any species 

 whatsoever is growing in its natural position. Still, there have 

 probably always been uncultivated wooded spots in the country to 

 which the Melica, Asperula, and Anemone could fly when any 

 particular wood was cut down, and from which they could settle 

 a new plantation. 



Who could trace a species through these changes of place, even 

 in one country "? It is obvious that it must be the proper habitat 

 and climate which limit their range, and nothing else. 



It follows, I think, that we may in some cases distinguish 

 indispensable conditions for the ranges of certain species. These 

 are as follows : — 



For halophytes — the existence of salt in the soil. 



For chalk and Limestone plants — geological boundaries, i.e., 



the presence of lime in the soil. 

 For peat plants — the presence of peat. 

 For Alpine plants — mountains of sufficient altitude, with a 



series of appropriate habitats. 

 For weeds of agriculture — agriculture carried on in a manner 



which varies with each individual weed. 

 For forage plants and those consciously introduced by man — 



a certain intelligence in agriculture, varying in a similar 



manner. 

 For water plants — the presence of running or stagnant 



water, with the required depth and bottom, and also 



with appropriate temperature. 

 For road-side, waste, shale, ballast, and railway plants — a 

 ,• ^ certain level of human civilisation, and also geological 



boundaries. 



