If one examines the large vessels going out from the heart of a lizard, he will find six aortic arches—i. e., three on each side. These all unite below to form the one descending abdominal aorta. This is shown in the accompanying figure ([Fig. 36]), in which a a′ a″ and b b′ b″ are the six arches. Now, there is no conceivable use in having so many aortic arches. We know this, because there is but one in birds and mammals, and the circulation is as effective, nay, much more effective in these than in reptiles. The explanation of this anomaly is revealed at once as soon as we examine the circulation of a fish, which is shown in the accompanying figure ([Fig. 37]). The multiplication of the aortic arches is here, of course, necessary, for they are the gill-arches. The whole of the blood passes through these arches, to be aërated in the gill-fringes. The use of this peculiar structure is here obvious enough. If a lizard were ever a fish, and afterward turned into a lizard, changing its gill-respiration for lung-respiration, then, of course, the useless gill-arches would remain to tell the story. Now, although a lizard never was a fish, in its individual history or ontogeny, it was a fish in its family history or phylogeny, and therefore it yet retains, by heredity, this curious and useless structure as evidence of its ancestry.
Fig. 38.
Fig. 39.
Figs. 38, 39.—Diagrams showing the change of the course of blood in the development of a frog. 38. The tadpole stage. 39. The mature condition. H, heart; G G′ G″, external gills; g g′ g″, internal gills; c c, connecting branches in the tadpole; p p, pulmonary branches.
That this is the true explanation is demonstrated by the fact that in amphibians this very change actually takes place before our eyes in the individual history. We have already seen that the individual frog, in its tadpole state, is a gill-breather. It has therefore its gill-arches ([Fig. 38]), three on each side, like a fish, and for the same reason, viz., the aëration of the blood. But when its gills dry up and lung-respiration is established, its now useless gill-arches still remain as aortic arches, to attest their previous condition ([Fig. 39]). Now, the lizard undoubtedly came from an air-breathing, tailed amphibian, and therefore inherited this form of arterial distribution. In both lizard and amphibian the ultimate cause is an origin from fishes, in which such arches are obviously necessary. The diagrams, [Figs. 38] and [39], are illustrations somewhat idealized, showing the manner in which the change actually takes place in air-breathing amphibians. [Fig. 38] represents the tadpole stage, and [Fig. 39] the mature condition. In the former the gills are mostly external, G G′, etc., but also internal, g g′, as in the fish. Observe in this condition the small connecting vessels, c c′. When the external gills dry up, these are enlarged, and the whole of the blood passes through them, as shown in Fig. 39. It is seen, also, in [Fig. 38], that a small branch, p, goes from the lower gill-arches to the yet rudimentary lung, l. When the gill-fringes have disappeared, the whole of the blood of the lower arch goes through the now enlarged pulmonary branch to the lungs, L, now in full activity, and the remainder of this arch disappears, as shown by the dotted lines in [Fig. 39].
The change which actually took place in the family history of the lizard probably differed from the above only in being more simple, the gills being only internal like the fish. The external gills complicate the process a little in the case of the frog, but the principle is precisely the same.
As already explained ([pages 82–85]), the large gap between fishes and reptiles, as regards mode of respiration, is completely filled both in the taxonomic series—i. e., in ganoids, dipnoi, and the mature condition of the different orders of amphibians—and in the ontogeny of the higher amphibians. Now, we add that the same is true of the arterial distribution. We have just traced the change in the ontogeny of the frog, but the steps of the same change are traceable in passing from the typical fish (teleosts), through dipnoi and amphibians to reptiles. Thus, again, the phylogeny, the taxonomy, and the ontogeny, are in complete accord.
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.