And now in our series of changing scenes comes all at once that strange blank which we hope one day to fill up; and when we look again the large reptiles are gone, the birds are spreading far and wide, and we come upon those early and primitive forms of insect-eaters, gnawers, monkeys, grass-feeders, and large flesh-eaters, whose descendants, together with those of the earlier marsupials, are henceforward to spread over the earth. We need scarcely carry our pictures much farther. We have seen how, in these early times, the flesh-feeders and grass-feeders were far less perfectly fitted for their lives than they are now;[198] how the horse has only gradually acquired his elegant form; the stag his branching antlers; and the cat tribe their scissor-like teeth, powerful jaws, and muscular limbs; while the same history of gradual improvement applies to nearly all the many forms of milk-givers. But there is another kind of change which we must not forget, which has been going on all through this long history, namely, alterations in the level and shape of the continents and islands, as coasts have been worn away in some places and raised up or added to in others, so that different countries have been separated from or joined to each other. Thus Australia, now standing alone, with its curious animal life, must at some very distant time have been joined to the mainland of Asia, from which it received its low forms of milk-givers, and since then, having become separated from the great battlefield of the Eastern Continent, has been keeping for us, as it were in a natural isolated zoological garden, the strange primitive Platypus and Echidna, and Marsupials of all kinds and habits. So too, Africa, no doubt for a long time cut off by a wide sea which prevented the larger and fiercer animals from entering it, harboured the large wingless ostriches, the gentle lemurs, the chattering monkeys, the scaly manis, and a whole host of insect-eaters; while South America, also standing alone, gave the sloths and armadilloes, the ant-bears, opossums, monkeys, rheas, and a number of other forms, the chance of establishing themselves firmly before stronger enemies came to molest them. These are only a few striking examples which help us to see how, if we could only trace them out, there are reasons to be found why each animal or group of animals now lives where we find it, and has escaped destruction in one part of the world when it has altogether disappeared in others. * * * * *