COMMON PLANTS. 309 
ture, and decreases with the diminution of heat.” This loose 
statement is sufficiently accurate for our purpose, viz. to show that 
species increase as they approach the equator, or they increase as 
the latitude decreases; and that they decrease as they recede 
from the equator and approach nearer to the pole, or decrease as 
the latitude increases, is a fact. The cause (the principal cause) 
is the increase of light and heat in the former case and its de- 
crease in the latter. It may be true that there is as much an- 
nual light in Lapland as in France, but there is a considerable 
difference between the annual heat of these two countries. The 
difference between the annual temperature in the south of Eng- 
land and that in the north of Scotland is only a few degrees. But 
this difference has a very material influence on plants. In these 
high northern latitudes few arboreous plants have time enough to 
ripen wood. The summers are too brief to admit of the forma- 
tion of buds before the leaves drop off. Herbaceous perennials 
are subject to similar laws: their vitality is resident in a radical 
' bud, but this cannot be formed till the plant has produced 
flowers and fruit, for which processes the seasons are too short in 
high latitudes. The constituents of the soil and the chemical 
composition of the plants themselves are not, when united, equal 
to the potent influence of atmospheric causes. The soil of the 
Land’s End and that of John o’ Groat’s are not surely very dis- 
similar. Neither is there a great difference in the character of the 
Salix caprea, that is produced equally in both. That grown in 
Cornwall is probably larger than its Caithness relative: to a bo- 
tanist they are the same, not individually, but specifically. It is 
a fact that fewer species are found about Cape Wrath than about 
Folkestone ; and we say that generally this is attributable to 
atmospheric causes, viz. a diminution of temperature. Moisture, 
one of the essential elements in promoting vegetation, is as plen- 
tiful in Caithness as in Kent. 
From the above-stated facts it is inferred that one-half of the 
plants spontaneously produced in the south of England are not 
present in the north of Scotland; and hence half the plants of 
Kent, Hampshire, or Devonshire are not common plants, as we 
have restricted the term common. It is well known to those 
who study such subjects that there are plants peculiar to Devon, © 
Hants, and Kent,—species found only in one or two of these 
counties, but not common to all three. Of course all such are 
