Of all known Vertebrate animals, the ancestors of the Primæval Fish probably showed most resemblance to the still living Sharks (Squalacei). They originated out of the single-nostriled animals by the division of the single nostril into two lateral halves, by the formation of a sympathetic nervous system, a jaw skeleton, a swimming bladder, and two pairs of legs (breast fins or fore-legs, and ventral fins or hind-legs). The internal organisation of this stage may probably, upon the whole, have corresponded to the lowest species of Sharks known to us; the swimming bladder was however more strongly developed; in the case of the latter it exists only as a rudimentary organ. They lived as early as the Silurian period, as is proved by the fossil remains of sharks (teeth and fin spines) from the Silurian strata. A certain proof that the Silurian ancestors of man and of all the other double-nostriled animals were nearest akin to the Selachii, is furnished by the comparative anatomy of the latter; it shows that the relations of organisation in all Amphirrhina can be derived from those of the Selachii.
Twelfth Stage: Mud Fish (Dipneusta).
Our twelfth ancestral stage is formed by Vertebrate animals which probably possessed a remote resemblance to the still living Salamander fish (Ceratodus, Protopterus, Lepidosiren, p. [212]). They originated out of the Primæval fish (probably at the beginning of the palæolithic, or primary period) by adaptation to life on land, and by the transformation of the swimming bladder into an air-breathing lung, and of the nasal cavity (which now opened into the cavity of the mouth) into air passages. The series of the ancestors of man which breathed air through lungs began at this stage. Their organisation may probably in many respects have agreed with that of the still living Ceratodus and Protopterus, but at the same time may have been very different. They probably lived at the beginning of the Devonian period. Their existence is proved by comparative anatomy, which shows the Dipneusta to be an intermediate stage between the Selachii and Amphibia.
Thirteenth Stage: Gilled Amphibians (Sozobranchia).
Out of those Mud Fish, which we considered the primary forms of all the Vertebrata which breathe through lungs, there developed the class of Amphibia as the main line (pp. [205], [216]). Here began the five-toed formation of the foot (the Pentadactyla), which was thence transmitted to the higher Vertebrata, and finally also to Man. The gilled Amphibians must be looked upon as our most ancient ancestors of the class of Amphibia; besides possessing lungs they retained throughout life regular gills, like the still living Proteus and Axolotl (p. [218]). They originated out of the Dipneusta by the transformation of the paddling fins into five-toed legs, and also by the more perfect differentiation of various organs, especially of the vertebral column. In any case they existed about the middle of the palæolithic, or primary period, possibly even before the Coal period; for fossil Amphibia are found in coal. The proof that similar gilled Amphibians were our direct ancestors, is given by the comparative anatomy and the ontogeny of Amphibia and Mammals.
Fourteenth Stage: Tailed Amphibians (Sozura).
Our amphibious ancestors which retained their gills throughout life, were replaced at a later period by other Amphibia, which, by metamorphosis, lost the gills which they had possessed in early life, but retained the tail, as in the case of the salamanders and newts of the present day. (Compare p. [218].) They originated out of the gilled Amphibians by accustoming themselves in early life to breathe only through gills, and later in life only through lungs. They probably existed even in the second half of the primary, namely, during the Permian period, but possibly even during the Coal period. The proof of their existence lies in the fact that tailed Amphibians form a necessary intermediate link between the preceding and succeeding stages.
Fifteenth Stage: Primæval Amniota (Protamnia).
The name Protamnion we have given to the primary form of the three higher classes of Vertebrate animals, out of which the Proreptilia and the Promammalia developed as two diverging branches (p. [222]). It originated out of unknown tailed Amphibia by the complete loss of the gills, by the formation of the amnion, of the cochlea, and of the round window in the auditory organ, and of the organs of tears. It probably originated in the beginning of the mesolithic or secondary period, perhaps even towards the end of the primary, in the Permian period. The certain proof that it once existed lies in the comparative anatomy and the ontogeny of the Amniota; for all Reptiles, Birds, and Mammals, including Man, agree in so many important characteristics that they must, with full assurance, be admitted to be the descendants of a single common primary form, namely, of the Protamnion.