might have existed when the ice passed away for recolonization across a land surface from the east, since a comparatively small elevation would connect the British Islands with the Continent. But that such an elevation continued for long after the passing of the ice is by no means certain. On the whole, the evidence of general glaciation of our islands as interpreted by geologists almost postulates the extinction within our area of the whole existing flora and fauna, and consequently its reconstruction by immigration when a temperate climate returned. But there is a body of evidence to be drawn from the present and past distribution of the existing plants and animals which is of great importance in this connection. Is this biological testimony in favour of the theory of the immigration of our flora and fauna during the relatively short period which has elapsed since the passing of the ice? To this question different observers have given very different answers. In order to form an idea of the nature of the problem--it is possible here to deal only with the case of the plants--we need to study briefly the composition of the present flora, from the point of view of its origin. In the first place, it must be recalled that the British Isles are situated on a broad shelf which extends into the Atlantic on the western edge of Europe. In comparison with the depth of the adjoining ocean, this shelf is but little below sea-level, and a slight elevation of the land--much smaller than those which have occurred over and over again in recent geological times--would join our islands to Germany, Holland, Belgium, and France. The British Isles are geographically and biologically by no means a separate area, and they have derived their population, both plant and animal, by immigration at various periods of time from the great land area to the eastward. Our present flora proves the truth of this as a general assertion; a study of its constituents shows that it is essentially a reduced continental European flora. As we step from France across to England we lose a number of plants familiar on the French side. As we step again from England into Ireland a further number of plants disappears; and these losses are no doubt due either to an unsuitability of climate on the insular areas, especially the absence of a hot summer, or to the inability of the plants to cross the barriers of sea which have now existed for some time. If the whole of the flora fitted in with this idea of mere reduction of the Continental flora by elimination, the problem would be much simplified. But there are other elements in it which do not harmonize with this conception of simply a general western migration, and which give rise to very interesting problems.