PERIODICITY IN ORGANIC LIFE.
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considered as due to the agency of man, to what are we to attribute
the abundance of this bird during the last few years ? I am not
aware that man has destroyed any of its enemies, nor am I aware
that he has cultivated any fresh crop which can have been utilized
by it for food ; nor do I know that meteorological conditions have
been peculiarly favourable. The only way of explaining it is by
stating the fact that this species is passing through its period of
abundance.
The Goldfinch is an instance of a species passing through, one
must hope, a period of scarcity only and not of approaching
extinction. The agency of man may possibly have some effect in
producing a portion of this rarity, but he certainly is not the sole
cause.
Amongst the migratory birds we often see a great abundance at
times, sometimes for two or three or more years in succession.
There may have been in the district from which they came abund-
ance of food or other favourable causes, which may account for
their increase, and therefore for the large numbers of our annual
visitors. Sometimes a greater degree of cold would appear to be
the explanation; but we must remember there are cold seasons
when the flocks are few, so that cold weather cannot be the only
factor. I have no doubt that if we could get a correct estimate for
every part of the globe visited by these northern breeders, we should
find the same inexplicable periodicity of abundance and the reverse
obtaining as the rule with these creatures as with all the classes we
have been considering; and that the numbers we see have nothing
to do with either heat, cold, or abundant food.
The next great division of nature, Mammalia, will furnish us with
many examples of the law of periodicity, and we shall in these be
able to follow the various apparent means by which the abundance
or scarcity is occasioned; but we cannot tell even then how these
means were brought about. This we can do from the more
stationary habits of the creatures, and from the large number under
the control of man.
Plenty, as regards food and favourable weather, will be found
very active agents in the welfare of, and abundance of, this class;
but meteorological agents, as far as we can see, have but little
influence. This I will illustrate by reference to a domestic animal
first. The expression "a good fall of lambs" is well known, and
it is a matter of observation that there are seasons when a very