The Evolution of Man — Volume 2 by Haeckel, Ernst Heinrich Philipp August, 1834-1919

Ceratodus is particularly interesting on account of the primitive build of its skeleton; the cartilaginous skeleton of its two pairs of fins, for instance, has still the original form of a bi-serial or feathered leaf, and was on that account described by Gegenbaur as a "primitive fin-skeleton." On the other hand, the skeleton of the pairs of fins is greatly reduced in the African dipneust (Protopterus) and the American (Lepidosiren). Further, the lungs are double in these modern dipneusts, as in all the other air-breathing vertebrates; they have on that account been called "double-lunged" (Dipneumones) in contrast to the Ceratodus; the latter has only a single lung (Monopneumones). At the same time the gills also are developed as water-breathing organs in all these lung-fishes. Protopterus has external as well as internal gills.

The paleozoic Dipneusts that are in the direct line of our ancestry, and form the connecting-bridge between the Ganoids and the Amphibia, differ in many respects from their living descendants, but agree with them in the above essential features. This is confirmed by a number of interesting facts that have lately come to our knowledge in connection with the embryonic development of the Ceratodus and Lepidosiren; they give us important information as to the stem-history of the lower Vertebrates, and therefore of our early ancestors of the paleozoic age.

CHAPTER 2.22. OUR FIVE-TOED ANCESTORS.

With the phylogenetic study of the four higher classes of Vertebrates, which must now engage our attention, we reach much firmer ground and more light in the construction of our genealogy than we have, perhaps, enjoyed up to the present. In the first place, we owe a number of very valuable data to the very interesting class of Vertebrates that come next to the Dipneusts and have been developed from them--the Amphibia. To this group belong the salamander, the frog, and the toad. In earlier days all the reptiles were, on the example of Linne, classed with the Amphibia (lizards, serpents, crocodiles, and tortoises). But the reptiles are much more advanced than the Amphibia, and are nearer to the birds in the chief points of their structure. The true Amphibia are nearer to the Dipneusta and the fishes; they are also much older than the reptiles. There were plenty of highly-developed (and sometimes large) Amphibia during the Carboniferous period; but the earliest reptiles are only found in the Permian period. It is probable that the Amphibia were evolved even earlier--during the Devonian period--from the Dipneusta. The extinct Amphibia of which we have fossil remains from that remote period (very numerous especially in the Triassic strata) were distinguished for a graceful scaly coat or a powerful bony armour on the skin (like the crocodile), whereas the living amphibia have usually a smooth and slippery skin.