Now something of this kind takes place with all that great group of animals we are going to study. The cat and the cow, as we all know, do not lay eggs as birds do; but the mother carries the young within her body while they are going through all the changes which the chicken goes through in the egg. Thus they go wherever she goes, the food which she takes feeds them, and they lie hidden, safe from danger, till they are born, perfectly formed, into the world. Nor is this all; for when at last her little ones see the light, the mother has nourishment ready for them; part of the food which she herself eats is turned into milk, and secreted by special glands, so that the newly-born calf or kitten is suckled at its mother’s breast till it has strength to feed itself. These two advantages, then,--namely, that the young have no dangerous egg-stage, but are sheltered by their mother till they are perfect, and that their mother has milk to give them for food,--at once divide the _Mammalia_ or milk-giving group of animals from the rest of the backboned family. But how will this help us to learn where that great group begins? Is it possible that such creatures as these can have anything in common with reptiles and birds? To answer these questions we must travel to a part of the world which has long been separated from the great continents of Europe, Asia, Africa, and America, and where the low and feeble milk-giving animals had a chance of still keeping a place in the world. Take a map and look at Australia, New Guinea, and Tasmania, and you will see that they are separated by a number of scattered islands from the great continents, which are not only large in themselves, but are all nearly joined together, with only narrow straits dividing them. Moreover, Australasia stands even more alone than appears at first sight; for Mr. Wallace has pointed out that a very deep sea separates New Guinea and Australia on the one hand from Borneo and China on the other; so that the land might rise several thousand feet, and yet the Australasian islands would not be joined to the great continents. Now, if the milk-givers once had feeble beginnings, and gradually branched out, as the ages went on, into all the many forms now living, it is clear that on the great battlefield of Europe, Asia, Africa, and America, the first poor weak forms would gradually be destroyed by the stronger ones that overran these great continents. They would be crushed out, as so many of the reptiles and newts and fishes had been before them; and only their bones, if any remained, would tell us that they had once lived. But if some of them could find a refuge in a domain of their own, where after a time they had a good open sea between them and their stronger neighbours, they might have a chance of living on and keeping up the old traditions.