ome
previous experience of reproduction? Well, we take a particular locality
and note the migrants that visit it year after year, and we find that
the respective numbers of the different species are subject to wide
annual fluctuations. Not every species lends itself to an inquiry of
this kind: some are always plentiful and fluctuation is consequently
difficult to discern; others are scarce and variation is easily
determined. Those which are of local distribution but conspicuous by
their plumage, or easily traced by the beauty or the peculiarity of
their song, afford the more suitable subjects for investigation. For
example, the Grasshopper-Warbler, Marsh-Warbler, Nightingale, Corncrake,
Red-backed Shrike, or Whinchat have each some distinctive peculiarity
which makes them conspicuous, and each one is subject to marked
fluctuation in numbers. The small plantation or wooded bank may hold a
Nightingale one year, but we miss its song there the next; the osier bed
or gorse-covered common which vibrates with the trill of the
Grasshopper-Warbler one April is deserted the following season; the
plantation which is occupied by a host of common migrants this summer
may be enlivened next year by the song of the rarer Marsh-Warbler also;
and so on. The fluctuation is considerable: we observe desertion on the
one hand, appropriation on the other, and yet males appear before
females whether the particular plantation, osier bed, or swamp had been
inhabited or not the previous season. This fact is not without
significance. It shows that similar conditions prevail both amongst the
males that appropriate breeding grounds new to them, and amongst those
that return to some well-established haunt; and on the assumption that
the earlier arrivals are experienced males, the same birds evidently do
not return to the same place year after year. Granting, then, that the
males which appropriate new breeding-grounds are young birds, how can
their earlier arrival be explained in terms of past experience; and
granting that they are old, and therefore experienced, how can it be
explained in terms of association?

Again, it may be urged that if there is some biological end to be
furthered by this hurried return, and if recollection of past experience
is a means towards that end, such recollection need not necessarily be
associated with a definite place, but only in a vague way with the whole
series of events leading up to reproduction--in which series t