LATITUDE ia 
ground, especially in wooded ravines. With regard to the 
alpine species, the ecological conditions in the north of 
Scotland are unfavourable for their presence. There are, 
however, two or perhaps three species which occur in the 
Highlands on hills of a similar height to those in the Low- 
lands, and the apparent absence of which from the latter is 
probably due to latitude. The ecological conditions are 
likewise unfavourable in the extreme northern islands of 
Orkney and Shetland, so much so that no alpine species 
have been found on the former and few in the latter. 
Those which are found in Shetland descend to a lower level 
than on the mainland, but the lower limit of altitude does 
not appear to be appreciably affected by latitude on the 
mainland except to a very limited extent. The main factor 
which affects the lower limits on the mainland is that the 
ecological conditions on the West Highland coast are favour- 
able for such downward extension, as is also the case with 
other countries of similar features, and this also applies to 
other forms of vegetation. 
Our climate being insular, there is little difference 
between the extremes of temperature due to latitude, but 
there is a considerable difference on account of longitude. 
Along the whole length of the western seaboard there is 
hardly any appreciable difference of winter temperature, 
and this temperature considerably exceeds the winter 
temperature of the centre and east of the whole of Scotland, 
and, it may be added, of the similar districts in England. 
On the other hand, the summer temperature is higher, and 
there is a greater amount of sunshine, in the centre and 
east of Scotland in corresponding latitudes than there is on 
the west, and the amount decreases with latitude. We 
should thus expect to find that Southern species, which 
require greater summer heat, would reach a higher latitude 
on the east and in the centre of the country than on the 
west, and this is mostly the case; also that the Southern 
species which are affected by latitude in our country mostly 
belong to the thalloid group, which are dependent on a con- 
siderable amount of summer heat in the soil. 
