In the plant world, substitution often takes place far more rapidly.  I doubt whether the stinging nettle, which renders picnicking a nuisance in England, is truly indigenous; certainly the two worst kinds, the smaller nettle and the Roman nettle, are quite recent denizens, never straying, even at the present day, far from the precincts of farmyards and villages.  The shepherd’s-purse and many other common garden weeds of cultivation are of Eastern origin, and came to us at first with the seed-corn and the peas from the Mediterranean region.  Corn-cockles and corn-flowers are equally foreign and equally artificial; even the scarlet poppy, seldom found except in wheat-fields or around waste places in villages, has probably followed the course of tillage from some remote and ancient Eastern origin.  There is a pretty blue veronica which was unknown in England some thirty years since, but which then began to spread in gardens, and is now one of the commonest and most troublesome weeds throughout the whole country.  Other familiar wild plants have first been brought over as garden flowers.  There is the wall-flower, for instance, now escaped from cultivation in every part of Britain, and mantling with its yellow bunches both old churches and houses and also the crannies of the limestone cliffs around half the shores of England.  The common stock has similarly overrun the sea-front of the Isle of Wight; the monkey-plant, originally a Chilian flower, has run wild in many boggy spots in England and Wales; and a North American balsam, seldom cultivated even in cottage gardens, has managed to establish itself in profuse abundance along the banks of the Wey about Guildford and Godalming.  One little garden linaria, at first employed as an ornament for hanging-baskets, has become so common on old walls and banks as to be now considered a mere weed, and exterminated accordingly by fashionable gardeners.  Such are the unaccountable reverses of fortune, that one age will pay fifty guineas a bulb for a plant which the next age grubs up unanimously as a vulgar intruder.  White of Selborne noticed with delight in his own kitchen that rare insect, the Oriental cockroach, lately imported; and Mr. Brewer observed with joy in his garden at Reigate the blue Buxbaum speedwell, which is now the acknowledged and hated pest of the Surrey agriculturist.

The history of some of these waifs and strays which go to make up the wider population of Britain is indeed sufficiently remarkable.  Like all islands, England has a fragmentary fauna and flora, whose members have often drifted towards it in the most wonderful and varied manner.  Sometimes they bear witness to ancient land connections, as in the case of the spotted Portuguese slug which Professor Allman found calmly disporting itself on the basking cliffs in the Killarney district.  In former days, when Spain and Ireland joined hands in the middle of the Bay of Biscay, the ancestors of this placid Lusitanian mollusk must have