424 DR. E. J. SALISBURY ON THE 
This relation to reaction probably accounts for the rarity of earthworms in 
some siliceous areas. According to Darwin, for instance, earthworms are 
practically absent from the Welsh hills, where the soils most commonly 
exhibit an acid reaction. Similarly, earthworms are apparently infrequent 
in the very alkaline peat of true fens, though they may be present in 
considerable numbers in fen peat from which the alkaline salts have been 
partially leached, or in fen peat of naturally mild reaction. 
But acidity or alkalinity are by no means the only factors which govern 
the distribution of earthworms, and amongst other conditions the proportion 
of organic material and the water content of the soil would seem most 
important. Darwin regarded Henson’s figure (Zeitsch. für Wiss. Zool. 
Bd. xxviii. 1877) of nearly forty thousand worms per acre as very exceptional, 
but much higher frequencies have been observed. ‘Thus in a locality 
with nearly neutral reaction, moderately high water content, and a high 
organic content the writer found a frequency of nearly 700,000 earthworms 
per aere. Some little-known observations of Dr. Brett may be quoted in 
this connection (Trans. Herts. Nat. Hist. Soc. 1883). This observer counted 
the worms in an area of one square yard of soil, dug out to a depth of three 
feet, in four different locations in a garden. The figures obtained range 
from sixty to one hundred and eighty earthworms per cubic yard, The latter 
number represents about 870,000 earthworms per acre, and it is pertinent to 
note that the location was a vine border where an approximately neutral 
reaction and a high organie content are essential to successful production. 
As already indicated, an increase of organic content is frequently accom- 
panied by an increased acidity and is usually correlated with a high water 
content. Despite, however, the suitability of the habitat in two of these 
respects, we find the reaction factor apparently dominating the frequency of 
the earthworms. Acidity (or alkalinity) would then appear to be a ‘ master 
factor” in the distribution of these animals. It is significant in this con- 
nection that a very marked increase in the number of earthworms was observed 
to follow the application of lime to a garden soil well supplied with organic 
material but distinetly acid in reaction. 
The writer has suggested that all natural soils tend to become more and 
more acid with increasing age, and this edaphic succession is accompanied 
by changes in the character of the vegetation (cf. Journal of Ecology, vol. ix. 
1922). The fact that earthworms do not occur where the subsurface is 
appreciably acid shows that their effect on this edaphic succession is to cause 
a retardation in the establishment of the reaction gradient, but it does not 
appear probable that the increasing acidity of the surface can be permanently 
checked by their influence. This conclusion seems to be indicated by the 
much more acid character of wormeasts on acid soils than on those which are 
less acid or neutral. Further work may, however, show that a given species 
produces wormcasts of a much narrower range of reaction. 
