PERIODICITY IN ORGANIC LIFE.
63
may be some truth in this notion, but the destruction of owls, etc.,
is probably not the only cause of the increase of the voles. In other
places they are increasing, for instance in Greece. Does strict game
preserving obtain in that kingdom, and are all hawks, owls, and other
mice-destroying animals exterminated there ? If they are not, then
we cannot say game preserving and the destruction of the carnivo-
rous birds and animals of Scotland has brought about the plague of
mice, as Greece is also suffering in a similar and probably more
extensive scale, from the same mischievous little beasts.
Again, in 1580 I do not think any great progress had been made
in Essex towards reducing the numbers of the mice-eating birds and
animals; still the pest appeared, got gradually worse, and at last
began gradually again to diminish, until only about the normal
numbers remained. It may perhaps be well to try and diminish the
number of these creatures in Scotland, but I think man will be able
to do but little in effecting this desirable object, and in my opinion
they might as well be left alone, as the laws which govern this
periodicity in abundance and scarcity will bring about most surely
their usual results, and only a proper number of these animals will
be found in the course of a few seasons. This diminution has
always occurred wherever the pest has prevailed.1
I will take the next illustration from a foreign rodent, the
Lemming. At uncertain intervals these animals abound to a mar-
vellous extent, and then commences a migration, the details of which
are very extraordinary, but we need not go into these, except so far
as they throw light on our subject.
It would appear that this animal is very common throughout the
Scandinavian peninsula, just as the Vole is in this country. After
a period (which may be long or short) it is noticed that there occurs
a considerable increase in the animal's numbers. This multiplication
goes on for some time, and at last it is found that the Lemmings are
evidently moving on, it may be towards the west. Nothing will turn
them from their course; even a river or a lake will not stem the
migration. It must not be supposed that they assemble and start
off all at one time, as the swallow does, and so complete their
journey as quickly as possible. It would rather appear that they
1 Since writing this paper, an article by the Editor has appeared in the "Zoologist" for
April 1893, not only confirmatory of the views I have expressed as to the influence of mice-eating
birds and mammals, but also, what is perhaps more interesting, bearing out the idea previously
mentioned that probably a larger fecundity may have had some influence in producing their
abundance. The writer states that Arvicola agrestis normally produces from four to six young
at a birth, but at the present time in the districts of Scotland suffering from this plague of mice
litters of eight or ten are very common—the equivalent of "a good fall of lambs."