314 Vn. AREAS OF SPECIES. 



leading to several alterations, curtailing the range of latitude for some 

 species, extending it for others. 



It is somewhat arbiUary to include among the "Austral plants" seve- 

 ral of those which are distributed nearly or quite throughout Britain, 

 some of them even to Orkney or Shetland. A practical convenience, pre- 

 sently to be explained, has induced this wide extension of the first 

 division of the list, so as to make it include various species which might 

 otherwise as truly and correctly have been enumerated under the second 

 division, thai of " General species." It will be observed that none of the 

 plants in this division, which are entered as reaching beyond the latitu- 

 dinal line of 57 northward, are noted for all the provinces ; some one or 

 more being invariably left blank, as shown by the dots, instead of the 

 figures which denote the several provinces. 



The Temperatures indicated as part of the head-titles for the groups 

 under this division, are not to be taken as rigidly exact. The tempera- 

 ture given is in each case the assumed sea-level temperature for the cor- 

 responding line of latitude, in accordance with the scales before given 

 on pages 156 and 158. It is thus a temperature below which the 

 species of that group are not observed to grow in Britain, if plants 

 found at or near the coast-level only. If occurring much above the 

 coast-level, the lowest temperature in which they grow is of course pro- 

 portionately lower. The temperature, above which the several plants 

 are not seen to grow, may be considered that of the latitudinal line or 

 degree given for their south limit by the figures set close after their 

 names. If not given,— that is to say, for plants extending southward of 

 latitude 51, — the temperature may be considered to rise I degree of 

 Fahrenheit, or 0.6 of centigrade, for half a degree of latitude ; being 51 

 or 52 Fahrenheit, and 10.6 or 11.2 centigrade, accordingly as the plants 

 are found to grow or not to grow further south than 50J. 



But in thus applying the two scales of Fahrenheit and centigrade, it 

 is to be recollected, the rates of decrease in temperature for increase of 

 latitude, or conversely, were not assumed quite equally. In conse- 

 quence, the mean temperatures indicated by the two scales for the 

 groups of plants, or for the lines of latitude under which lliey are placed, 

 are not strictly even and correspondent the one with the other, except at 

 the starting point, under the first heading of " 1. Southward of 51 lati- 

 tude," — where 50 fahr. and 10 cent, do precisely correspond. It is also 

 to be kept in memory, that among the plants ceasing somewhere be- 

 tween any two lines of latitude, — say, for illustration, between 52-53, — 



