But the argument for evolution does not stop here. If birds and mammals have come from reptiles, and therefore from fishes, we may expect to find some evidences of the same kind still lingering in the great arteries. And such we do find. It is a most curious and significant fact that, in the early embryonic condition of birds and mammals, including man himself, we find on each side of the neck several gill-slits, each with its gill-arch, and therefore _several aortic arches on each side_, precisely similar to what we have already described. These arches are subsequently, some of them, obliterated; some modified to form the one aortic arch, and some of them still more modified to form the other great arteries coming from the heart to supply the head and forelimbs. [Illustration: FIG. 40.--Diagram of mammalian heart. _a_, aorta; _p_, pulmonary artery; _scs′c′_, subclavium on each side; _cc′_, carotids on each side. ] [Illustration: FIG. 41.--Ideal diagram representing the primitive aortic arches (after Rathke).] This is so beautiful and convincing an example, and one so generally unfamiliar, to even intelligent persons, not especially acquainted with biology, that it is best to explain it more fully. In Fig. 40 we give a mammalian heart and outgoing vessels, very slightly modified, so as to suggest the process of change. In Fig. 41 we give an ideal diagram representing the primitive aortic arches as they exist in the embryo of mammals, birds, and reptiles. It represents, also, substantially, the arches as they exist in the _mature_ condition in the most reptilian fishes (dipnoi) and in some sharks, except that in these the arches are of course furnished with gill-fringes. We will use this figure, therefore, to represent both the embryonic condition of air-breathing vertebrates and the mature condition of some fishes. The place of the heart is indicated by the dotted circle. Fig. 36, on page 134, shows what these arches become in reptiles (lizard). It is seen that the two upper arches on each side are obliterated, as indeed they already are in some teleost fishes. Fig. 42 shows what they become in birds. The two upper arches are, of course, obliterated. The others are all modified, each in a manner which may be readily understood by comparison with Fig. 41. Finally, Fig. 43 shows what they become in mammals and in man. In the bird (Fig. 42) the first pair of arches become the two pulmonary arteries as they do also in the lizard. The second pair become on the right side (left of the diagram) the aortic arch, on the left side (right of the diagram) the left subclavian, _s′c′_ (the right subclavian, _sc_, is a branch of the aortic arch). The third pair become carotids, _cc_, while the fourth and fifth, as already said, are aborted. In the mammal (Fig. 43), on the left side (right of the diagram) the first arch becomes the pulmonary artery, _p_. In the fœtus the continuation of this arch forms the ductus