Before this colonisation was complete, however--that is, before all the different kinds of European beasts and birds had made their way to the extreme western districts--communication with the continent was broken off. The land of the north-western districts of Europe sank. The sea flowed in, forming the German Ocean, the English Channel and the Irish Sea, and the influx of animal life was stopped. This is the reason why there are more than twice as many kinds of land animals in Germany as there are in England, and nearly twice as many in England as there are in Ireland. This is the reason why there are no snakes in Ireland, and why the nightingale, on returning from the south, never crosses into the sister kingdom. On islands that have long been separated from a continent it is found that forms of life tend to vary in the lapse of time, and that fresh species are developed. That it is not long, as geological periods go, since Great Britain became an island, is shown by the fact that we have no quadruped or reptile except the Irish weasel (_Mustela hibernica_), and, setting aside minor differences which some writers have magnified to the value of a species, only one bird, the red grouse, which is not also to be found in Europe. Very different is the case in Japan, which was separated from the mainland of Asia so long ago that new species have had time to develope; and the islands of that country contain many kinds of beasts and birds which are unknown on the adjacent continent. Some of the animals which came from Europe into Britain have died out, either because the climate changed and so cut off their food supply, or because they were destroyed by the hunters of the Stone Age. The bones which have been found in Kent's Cavern at Torquay, and in other caverns, afford clear evidence that the mammoth, the lion, the bear, and the hyaena once roamed over the hills of Devonshire. Although there are many more species of beasts and birds on the continent of Europe than there are in this country, both birds and beasts are numerically much more common here. Nothing strikes a naturalist more forcibly when travelling in France or Italy, for example, than the scarcity of wild life, and especially the fewness of the birds. It is true that we have fewer species, but we have many more individuals. To this, several causes have contributed. Englishmen do not, as is the custom in many European countries, shoot or trap for food small birds of every description. And game preserving--although it has been fatal to the larger birds of prey, such as kites, falcons, and buzzards, and keeps down other species, such as jays, magpies, and carrion crows--provides innumerable sanctuaries for great numbers of the smaller birds, which are safe from harm during the breeding season.