In former days, when the mammoth or hairy elephant, the extinct woolly rhinoceros, and the wild ox, together with the African hippopotamus and spotted hyaena roamed over the Thames valley and afforded sport to our prehistoric ancestors, England was joined to the Continent across what is now the English Channel; so that the animals and plants of the southern portion of our islands, at any rate, were more or less nearly identical with those of France and Belgium. The advent of the great ice age, or glacial period, caused, however, a vast disturbance of the fauna and flora (as the assemblages of animals and plants characteristic of different countries are respectively termed), especially as about this time there occurred several oscillations in the level of our country, during one or more of which Great Britain was temporarily separated from the Continent. How much or how little these and other changes had to do with the poverty of the British fauna as compared with that of the Continent is too long and difficult a question to be discussed in this place; but certain it is that even the southern counties of England do possess fewer species of animals and plants than France or Belgium; that this poverty increases with the distance from the Continent; and that Ireland is much poorer in species than England. It may perhaps be well to add, although it does not really concern our subject, that there appears to have been another land-connection by means of which Scotland and Ireland received a portion of their faunas from Scandinavia by way of what is now the North Sea. [Illustration: Six Hills, Stevenage (Danish Barrows)] At the date of the final insulation of Great Britain from the Continent there is every reason to believe that all the land animals of the former were identical with species inhabiting adjacent regions of the latter. And even at the present day, when isolation has for centuries been exerting its influence on the non-migratory (and in some degree also on the migratory) animals, there are no species of quadrupeds (mammals), birds, or reptiles absolutely peculiar to our islands, with the exception of the grouse; and in the opinion of many naturalists that bird should be regarded rather as a local variety or race of the willow-grouse of Scandinavia than as a distinct species.