i6o F. E. Fritsch and Winifred M. Parker. 
This last point requires some further consideration. In view 
of the fact that the C U facies is largely confined to the top and 
upper parts of the central ridge, it might be supposed that the C U 
and CUE owed their distinction to the slightly different habitats 
or to soil-conditions. These possibilities have not been disregarded, 
but there are, we think, important objections to such a view, which 
may be briefly summarised as follows:— 
(i) The C U facies is not entirely confined to the top and 
upper slopes of the ridge, but at some points extends right down 
into the valleys (cf. p. 153 and the map). This is also true of other 
parts of the Hindhead Common, not included in the map (owing to 
their survey not being complete). Where the C U facies extends 
down into the valleys, an investigation of the soil has failed to show 
any difference from that inhabited by the adjacent CUE facies. 
(ii) If the two facies were due to the difference of habitat, 
one would expect to find a gradual transition between them, since 
the conditions of the habitat do not change suddenly. As it is 
however there is a very sudden change from the C U E to the C U, 
so sharp that it can easily be seen half-a-mile away (cf. PI. V, figs. 
1 and 2, and p. 154). We have no doubt that this sharp line of 
demarcation marks the limit of a fire, since the recent fires have 
left just such a line at other points on the area. 
(iii) The Calluna and Ulex in the C U facies are much older 
(10-14 years) than the Calluna and Ulex in the CUE (about 5 
years old). 
We think that these objections are sufficiently strong to make 
it very unlikely that the two facies are due to habitat or soil- 
conditions, and that they constitute good presumptive evidence for 
the view that the C U facies is merely a later stage of the CUE 
facies. Direct evidence is of course at present not obtainable, nor 
is it possible to say exactly how the transition is accomplished, 
But it appears probable that the dense branching of older 
Calluna -plants has a great deal to do with the elimination of the 
other constituents of the CUE facies. It may be recalled (p. 153) 
that where there is a break in the continuity of the Calluna on the 
ridge-top Erica and Vaccinium very often occupy the ground, and 
this tends to show that the habitat is quite suitable, but that these 
forms can only get on where they can escape the dense shade of 
the tall Calluna. As our observations progress year by year we 
hope to be able to collect the direct evidence, that will completely 
establish the correctness of the above view. 
