266 THE ENTOMOSTRACA OF EPPING FOREST.
variations in the characters of the faunas of different districts
from a mere enumeration of the species. But if the relative
abundance of the various members of the local faunas could also
be taken into account I feel convinced that some very striking
differences would be brought to light.
The exact determination of the abundance of any species is necessarily
out of the question, but rough and ready approximations of practical value
may be obtained by the simple method of recording (i) the total number of
times, and (2) the total number of stations at which each species has been
obtained in the area under consideration. If the collections from each
station were equal in number, then the first series of figures would directly
represent the relative abundance of the different species, and the second
saries the relative distribution, whilst by a comparison of the two we
should be able to make out several other important facts. Thus if in
the case of any particular species the number of records (A) were high
and the number of stations (B) also high, then the species must be an exceed-
ingly common one, for it is evidently very constant in its occurrence and at the
same time widely distributed. If, however, A were comparatively high and B
low, then the species would be restricted in its distribution, although very con-
stant in its own particular haunts. Again if A were comparatively low and B
high, then the species must be fairly well distributed but very inconstant in
its appearance. Finally if both A and B were low, then presumably the
species must be rare in the district. In connection with the latter point, how-
ever, it should be borne in mind that arguments based on small figures are
much more likely to be wrong than those based upon large ones.
But it is not always possible to collect regularly from each station, and
then some of the conclusions to be drawn from an examination of the two
series of figures mentioned are not quite the same as those given above. The
total number of records for instance of any given species does not necessarily
represent its abundance-value for the whole district, although it probably
does do so nine times out of ten: But in the tenth case a very high figure
representing the number of records, associated with a comparatively low
figure for the number of stations, or via versa, may be explained by the
fact of comparative over-collection in a few favoured stations and under-
collection in others, rather than by the exceptional constancy or inconstancy
in the appearance of the species as stated above. The only method under
these circumstances by which to get a single series of numbers repre-
senting approximately the relative abundance of the species in the whole
district would be to use the following formula (A / C) x B, where A and B have
the same values as before, and c equals the total number of collections made
at the particular stations at which each species has been found.
It would not be worth while in the present instance to calcu-
late a series of numbers in the manner suggested above, because
of the many imperfections in the records, in addition to that of
unequal collecting at the various stations. It will be much
simpler, and probably not very far wrong in the case of the com-
moner species, to consider that the total number of records repre-
sents a fair approach to an accurate measure of relative abun-