Thus, then, in ontogeny the fish goes no further than the fish stages. The perennibranch passes through the fish stage to the perennibranch amphibian. The caducibranch takes first the fish-form, then the perennibranch-form, and finally the caducibranch-form, but goes no further. Last, the anoura takes first the fish-form, then that of the perennibranch, then that of the caducibranch, and finally becomes anoura. This is shown in the diagram, which must be read upward, line by line.
FISH. PERENNIBRANCH. CADUCIBRANCH. ANOURA.
^
FISH. PERENNIBRANCH. CADUCIBRANCH.--------^
^
FISH. PERENNIBRANCH-----------^
^
FISH.------------^
Diagram showing the stages of development of amphibians. (To be read upward.)
Now, this is undoubtedly the order of succession of forms in geological times—i. e., in the phylogenic series. This series is indicated by the arrows in the diagram. Fishes first appeared in the Devonian and Upper Silurian in very reptilian or rather amphibian forms. Then in the Carboniferous, fishes still continuing, there appeared the lowest—i. e., most fish-like—forms of amphibians. These were undoubtedly perennibranchs. In the Permian and Triassic higher forms appeared, which were certainly caducibranch. Finally, only in the Tertiary, so far as we yet know, do the highest form (anoura) appear. The general similarity of the three series is complete. If we read the diagram horizontally, we have the ontogenic series; if diagonally with the arrows, we have both the taxonomic and the phylogenic series.
2. Aortic Arches.—But some will, perhaps, say that these stages in the ontogeny are only examples of adaptive modifications—like modifications for like conditions of life—and had better be accounted for in this way, without reference to family history. We will, therefore, take another example, which can not be thus accounted for—an example in which there is no possible use now for the peculiar form or structure which we find. For this purpose we take the case of the course of circulation in vertebrates.
Fig. 36.—Showing heart and outgoing blood-vessels of a lizard (after Owen). The arrows show the course of the blood.
Fig. 37.—A, heart and gill-arches of a fish; B, one arch, with fringe (after Owen); H, the heart.