Even since the insulation of Britain a great many new plants and animals have been added to our population, both by human design and in several other casual fashions. The fallow deer is said to have been introduced by the Romans, and domesticated ever since in the successive parks of Celt and Saxon, Dane and Norman. The edible snail, still scattered thinly over our southern downs, and abundant at Box Hill and a few other spots in Surrey or Sussex, was brought over, they tell us, by the same luxurious Italian epicures, and is even now confined, imaginative naturalists declare, to the immediate neighbourhood of Roman stations. The mediaeval monks, in like manner, introduced the carp for their Friday dinners. One of our commonest river mussels at the present day did not exist in England at all a century ago, but was ferried hither from the Volga, clinging to the bottoms of vessels from the Black Sea, and has now spread itself through all our brooks and streams to the very heart and centre of England. Thus, from day to day, as in society at large, new introductions constantly take place, and old friends die out for ever. The brown rat replaces the old English black rat; strange weeds kill off the weeds of ancient days; fresh flies and grubs and beetles crop up, and disturb the primitive entomological balance. The bustard is gone from Salisbury Plain; the fenland butterflies have disappeared with the drainage of the fens. In their place the red-legged partridge invades Norfolk; the American black bass is making himself quite at home, with Yankee assurance, in our sluggish rivers; and the spoonbill is nesting of its own accord among the warmer corners of the Sussex downs.