62 PERIODICITY IN ORGANIC LIFE.
large proportion of the lambs born are twins. There are other
years when twins are few. I have never been able to account for
this ; and on looking back to the previous season, there seems to
have been nothing remarkable ; a fine summer may have been
followed by few twins, and a cold unfavourable season may have
had many. In the matter of food, so purely an artificial animal
kept under the conditions as the sheep in this country is, knows
little of scarcity, and therefore this cannot much interfere with the
fruitfulness of the ewes.
There is another domesticated animal which gives us an example
of the contrary result, and shows us how periodicity in scarcity may
be produced.
During certain years, without any apparent difference in season
or food, it is noted throughout this kingdom that large numbers of
cows abort. This unfortunate accident is not confined to one farm
or one county, but is general throughout the kingdom. When a
season of this kind occurs, it must have a very considerable effect
on the number of the young to be raised; and, therefore, if these
animals were living in a state of nature, periodicity in scarcity would
be the result.
Something of this kind may occur amongst creatures not under
man's control, but from want of opportunity we cannot say that it
does or does not do so. Arguing from what we see and know of
our domestic animals, it is, I think, a fair inference that possibly
this temporary fruitfulness or unfruitfulness may be one of the causes
contributing to the period of abundance or scarcity throughout all
nature.
At various times for many years past, the Field Vole (Arvicola
agrestis) has become so numerous in the marshes of Essex, that the
whole of the grass has been eaten by them. The first mention of
this plague was in 1580. Since this time there are many records of
the destruction produced by these swarms of mice in these same
marshes. We now hear that the fields in the south of Scotland are
overrun with mice, to the serious detriment of the pasturage, and
that an application has been made to the Government for assistance
in ridding the district, which is a very large one, of these unwelcome
visitors or natives. Many theories are promulgated to account for
the vast numbers existing ; amongst others that the destruction of
so-called vermin has, by doing away with nature's means of checking
undue increase, brought about this sad state of things. There