a63 



Eight species, found chiefly in the hilly districts of the north and 

 west, yet occasionally occur " so far from the mountain tracts, that they 

 cannot be held in the character of exclusively mountain ferns." — 



Cistopteris alpina Hymenophyllura Wilsoni Polypodium calcareum 



dentata Polypodium Phegopteris Botrychium Lunaria 



fragilis Dryopteris 



It is next remarked that " under the combined influence of latitude 

 and longitude, thus modified by the effect of elevation of surface, the 

 lower limits of many ferns, equally as those of flowering plants, ap- 

 pear on a map like irregular lines, whose general direction runs from 

 south-west to north-east ; whilst their upper limits encircle the hills, 

 or the hilly tracts, like zones or belts." It must not, however, be 

 supposed that by terminal lines are to be understood any others than 

 " artificial lines, drawn on a map, so as to connect the extreme sta- 

 tions for any species in either direction." Also the terms upper and 

 lower limits are to be understood as applying " to latitude, to eleva- 

 tion above the sea-level, and also in some measure to the degree of 

 proximity to the mountain tracts." In the neighbourhood of the lat- 

 ter a great change in the character of the Flora of a district becomes 

 evident, although the latitude and absolute elevation remain nearly 

 the same. Thus the upper limit of British plants will include, unless 

 otherwise qualified, the three conditions of more northern latitude, in- 

 creased elevation and greater proximity to mountain tracts. Again, 

 by the lower limits are to be understood " the opposite conditions of 

 southern latitude, diminished elevation, and also comparative remote- 

 ness from the mountain tracts, as centres around which the species are 

 distributed." 



These explanations being kept in view, it will be seen to be impos- 

 sible at present to represent on maps the distribution of British plants 

 with anything like accuracy, " in consequence of the upper limits of 

 most of the species being yet so little known." Their upper limits in 

 latitude might be traced pretty accurately ; and an approximation to 

 their altitude above the sea-level might be arrived at ; but the diffi- 

 culty would be to determine their propinquity to the mountain cen- 

 tres. This can only be ascertained when botanists shall carefully re- 

 cord the places where plants of the plains are last seen by them, as 

 they enter amongst the valleys of the mountain districts. 



" As an example of such difficulties, let us take Scolopendrium vulgare, a fern 

 widely diffused in Britain from Orkney to the Isle of Wight, and abundant in the 

 south of England. Betwixt these extreme points, however, there are many wide spaces 



