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VIII] conditions: this can only be brought about by a constant invasion of species from all the surrounding regions. Some hold their own, most cannot; but as time goes on, the surviving assemblage consists more and more of species which have been able to fight against the severe competition and colonize a new country.

Garden experiments are of little use as tests of the capability of any plant to survive in this country; the study of cornfield weeds is no better. In both cases the cultivation of the land produces a bare place on which a foreign introduction has as good a chance as a native. But could this foreigner survive if the seed were dropped on a natural moor or meadow? In this connexion it is noticeable that great part of the rare British plants occur close to the coast, opposite the part of the continent in which they are found, though they are not maritime species. This is probably due to two different causes, both acting in the same direction. In the first place most of these local plants are obviously late comers, which have not yet had time to spread inland or far. And, secondly, on the coast alone do we find any considerable extent of natural bare land—practically garden land—which does not at the same time consist of poor soil. The tumbled undercliffs of our coast are just the places to give a foreign invader a chance; there only will it find patches of bare good soil, full of small cracks in which a seed is hidden from birds.