60 PERIODICITY IN ORGANIC LIFE.
this is required to account for periodic abundance and scarcity when
the area over which either may occur is very large.
For some few years, on our Essex and Suffolk coasts, sprats
have not been in their usual abundance, and, as far as known, there
is nothing to account for the scarcity. Their comparative scarcity
has not been due to either change of locality or over-fishing, and we
can only surmise that this fish is now passing through its period of
less abundance. It is to be hoped, for the sake of our seaside
dwellers, its period of abundance may soon return. Fish are most
difficult subjects for observation, and I will therefore simply pass them
over, noting only as a matter of common observation, we have bad
years in many species, that is years when few are taken.
Birds are much better subjects for study and observation, and
we find periodicity in abundance and scarcity well marked in many
species.
The first I will take is the House Martin. In many, in fact in
most, parts of the kingdom, we hear that this bird is becoming
rarer ever year, and the cause is said to be that the sparrow, being
so abundant, takes possession of the nests of the Martin, and so
prevents their increase. This may be one cause of the lessening
numbers, but I do not believe that it is the only one, for the Martin is
diminishing even in those parts where the sparrow is not, and never
has been, numerous. There must, therefore, be other causes; I do
not know that man has had anything more to do with it than have
the seasons. Everywhere in this kingdom the Martin is a favourite,
and as such is rarely interfered with; even on the Continent, where
bird slaughter is so common, I do not think that it is esteemed as
an article of food. From some cause this most interesting little
creature is diminishing in numbers; undergoing its period of scarcity,
"one hopes, but almost fearing that the facts point to approaching
extinction.
As an illustration of the opposite condition—a period of abund-
ance—we may take the Hawfinch (Coccothraustes vulgaris). It is
not many years since this species was esteemed rare, and the find
of a nest was almost unheard of. This was in its period of scarcity ;
but now, not only do we hear of its breeding in Epping Forest,
where Mr. Doubleday first recorded its nest as occurring, but also
in every part of this county its nest is frequently found. And to those
who know when and where to look for it there is generally no diffi-
culty in finding a specimen. Now, if the scarcity of species is to be