I have quoted this passage from Bentham because it emphasises a point
which Darwin for his purpose did not find it necessary to dwell upon,
though he no doubt assumed it. Dispersal to a distance is, so to speak,
an accidental incident in the life of a species. Lepidium Draba, a
native of South-eastern Europe, owes its prevalence in the Isle of
Thanet to the disastrous Walcheren expedition; the straw-stuffing of the
mattresses of the fever-stricken soldiers who were landed there was used
by a farmer for manure. Sir Joseph Hooker ("Royal Institution Lecture",
April 12, 1878.) tells us that landing on Lord Auckland's Island, which
was uninhabited, "the first evidence I met with of its having been
previously visited by man was the English chickweed; and this I traced
to a mound that marked the grave of a British sailor, and that was
covered with the plant, doubtless the offspring of seed that had adhered
to the spade or mattock with which the grave had been dug."

Some migration from the spot where the individuals of a species
have germinated is an essential provision against extinction. Their
descendants otherwise would be liable to suppression by more vigorous
competitors. But they would eventually be extinguished inevitably,
as pointed out by Bentham, by the exhaustion of at any rate some one
necessary constituent of the soil. Gilbert showed by actual analysis
that the production of a "fairy ring" is simply due to the using up
by the fungi of the available nitrogen in the enclosed area which
continually enlarges as they seek a fresh supply on the outside margin.
Anyone who cultivates a garden can easily verify the fact that every
plant has some adaptation for varying degrees of seed-dispersal. It
cannot be doubted that slow but persistent terrestrial migration has
played an enormous part in bringing about existing plant-distribution,
or that climatic changes would intensify the effect because they would
force the abandonment of a former area and the occupation of a new one.
We are compelled to admit that as an incident of the Glacial period a
whole flora may have moved down and up a mountain side, while only some
of its constituent species would be able to take advantage of means of
long-distance transport.

I have dwelt on the importance of what I may call short-distance
dispersal as a necessary condition of plant life, because I think it
suggests the solution of a difficulty which leads Guppy to a conclusion
with wh