As in the living Peripatus, therefore, we have here a very valuable connecting link between two very different types of organisms. It is clear that one of the smaller reptiles--the Archaeopteryx is between a pigeon and a crow in size--of the Triassic period was the ancestor of the birds. Its most conspicuous distinction was that it developed a coat of feathers. A more important difference between the bird and the reptile is that the heart of the bird is completely divided into four chambers, but, as we saw, this probably occurred also in the other flying reptiles. It may be said to be almost a condition of the greater energy of a flying animal. When the heart has four complete chambers, the carbonised blood from the tissues of the body can be conveyed direct to the lungs for purification, and the aerated blood taken direct to the tissues, without any mingling of the two. In the mud-fish and amphibian, we saw, the heart has two chambers (auricles) above, but one (ventricle) below, in which the pure and impure blood mingle. In the reptiles a partition begins to form in the lower chamber. In the turtle it is so nearly complete that the venous and the arterial blood are fairly separated; in the crocodile it is quite complete, though the arteries are imperfectly arranged. Thus the four-chambered heart of the bird and mammal is not a sudden and inexplicable development. Its advantage is enormous in a cold climate. The purer supply of blood increases the combustion in the tissues, and the animal maintains its temperature and vitality when the surrounding air falls in temperature. It ceases to be "cold-blooded." But the bird secures a further advantage, and here it outstrips the flying reptile. The naked skin of the Pterosaur would allow the heat to escape so freely when the atmosphere cooled that a great strain would be laid on its vitality. A man lessens the demand on his vitality in cold regions by wearing clothing. The bird somehow obtained clothing, in the shape of a coat of feathers, and had more vitality to spare for life-purposes in a falling temperature. The reptile is strictly limited to one region, the bird can pass from region to region as food becomes scarce.