Next to this north-and-south change, due to climate, we may notice an east-and-west change, due partly to climate, but more perhaps to elimination, for in passing from France to Ireland we have to cross two barriers of sea. The climatic change is not unlike that experienced in going from south to north. We leave a dry climate (rainfall under 25 inches at year) for one of increasing wetness, a warm for a cool summer, a colder for a milder winter. The chief difference between the extreme west of the British Isles and the extreme north lies in the warmer winter of the former, frost being almost unknown in the milder spots. But the general similarity of northern and western conditions as opposed to eastern and southern leads to a fusing of the northern and western plant groups, so that on a map designed to show the distribution of our species analyzed according to their general range in Europe, the grouping of plants in the British Isles will be found to be roughly north-western as opposed to south-eastern. The further change due to elimination of species has been already referred to. Most plants no doubt have spread in our islands as far as prevailing climatic and soil conditions allow, but in other cases the sea-barriers seem to have put a period to their natural advance. Considering the wide range of conditions of climate and soil under which, for instance, the Hairy Crowfoot (_Ranunculus sardous_), the Common Rock-rose (_Helianthemum Chamæcistus_), the Needle Furze (_Genista anglica_), and the Small Marsh Valerian (_Valeriana dioica_), occur in England, Wales, and Scotland, it is difficult to impute their absence from Ireland to climate. Thirdly, we find (as we have already seen in the first chapter) varying conditions of soil intruding themselves and producing such local changes in the grouping of the plants as may quite obscure the broader differences just dealt with. Were our islands a plain formed of uniform materials, the gradual changes from south to north or east to west might be traced step by step. But their surface is most diversified; their rocks contain an epitome of the whole geology of Europe; the soils are consequently various: from the point of view of the plant world the area is an archipelago: for some plants a desert with occasional oases, for others an oasis enclosing occasional deserts. Certain species are confined to the Chalk--for instance, the Box (_Buxus sempervirens_) and the Stinking Hellebore (_Helleborus fœtidus_)--while to others a limy soil is a barrier comparable to that formed by the English Channel. It will be seen, then, that when we speak of the flora in general being a reduced Continental one, many considerations, geographical, climatic, and edaphic, must be duly taken into consideration if we are to understand the composition and distribution of our vegetation.