Reaction and Succession in Relation to the Plant Covering, 407 
The amount of organic material contained in these rabbit droppings is 
important, as also their reaction. With respect to the former point numerous 
counts were made of the number of rabbit droppings on a surface of unit 
area, and also of those present on and below the surface of a unit volume 
(cf. Table VIII). The figures for the surface deposit were also checked by 
entirely denuding an area of 50 sq. metres of all the rabbit faeces visible to 
the unaided eye (cf. below). 
It will be realized from the data in Table VIII that rabbits play 
no small part in the supply of organic material to the soil. 
Comparing the figures for the open dune, and those for areas of 
the same dune from which rabbits had been excluded for a period of 
3J years, we arrive at the conclusion that the annual deposit (assuming the 
rate of decay of the old droppings to be of the same order as for old and 
new together in the unenclosed area) is approximately 34 grm. on an area of 
312 sq. cm,, or about 0-18 per cent, by weight of the soil. 
A striking feature of the decay of these faeces is that their form is more 
or less retained throughout the process, so that one can roughly grade 
them according to age by the change in size, as well as by the progressively 
darker tint. 
Estimations of the hydrogen-ion concentration show that as decay pro- 
ceeds there is a similar change with age as observed for normal plant 
remains, the early stages being much more acid, p. H. 5*8-p. H. 6- 4 
(av. p. H. 6*i), than the most advanced state of decay (av. p. H. 6-9). 
Despite this difference, however, and the comparatively low buffer action of 
sand dune soils, there is only a slight difference in the reaction of the 
enclosed and unenclosed areas. Such difference as exists is indeed the 
reverse of what one might expect from the absence of recent droppings in 
the enclosed areas (cf. Table IX). 
The clear tendency, as shown in Table VIII, 1, is for the rabbit drop- 
pings to decrease in amount from the younger to the older dunes (compare 
‘ yellow ’ and ‘ grey ’ dunes, Table VIII) in correspondence with the diminish- 
ing amount of Psamrna arenaria and other of the more favoured food 
plants. 
Table IX. 
Area A . 
Enclosed (Sample I) 
p. H. 6-85 
Unenclosed (Sample I) 
p.H. 6.95 
» ( „ II) 
„ 6.85 
„ ( „ II) 
n 6-95 
„ ( „ HI) 
„ . 6.80 
„ ( „ HI) 
» 7*°5 
Area B. 
Enclosed (Sample I) 
p.H. 6.9 
Unenclosed (Sample I) 
p.H. 7.0 
>. ( id 
» 6 *9 
„ ( „ 11 ) 
„ 6.9 
( „ hi) 
„ 6‘9 
„ ( „ .HI) 
>, 6.9 
Av. enclosed p. H., 6*86 ; av. unenclosed p. H., 6*95 
