NATURAL HISTORY 27 extreme low water. Between England and the Continent the sea is very shallow, and St Paul's Cathedral might be placed anywhere in the North Sea without submerging its summit, but a little west of Ireland we soon come to very deep soundings. Great Britain and Ireland were thus once part of the Continent and are examples of what geologists call recent continental islands. But we also have no less certain proof that at some anterior period they were almost entirely submerged. The fauna and flora thus being destroyed, the land would have to be re- stocked with animals and plants from the Continent when union again took place, the influx of course coming from the east and south. As, however, it was not long before separation occurred, not all the continental species could establish themselves. We should thus expect to find that the parts in the neighbourhood of the Continent were richer in species and those furthest off poorer, and this proves to be the case both in plants and animals. While Britain has fewer species than France or Belgium, Ireland has still less than Britain. Small though England may be, she can nevertheless show most striking differences of fauna and flora in different districts. On the moors of the north, for example, the heaths and berries underfoot, and the larger birds of prey and grouse which now and again meet our view offer a marked contrast to let us say the furze- clad chalk downs of Sussex, where the wheatear and whinchat and the copper butterflies and " blues " are familiar objects. These differences depend upon a number of conditions, often mutually interdependent upon varia-
