| Geological Periods | Ancestral Stem-groups | Living Relatives of Ancestors |
| Silurian |
16. Selachii Primitive fishes Proselachii | 16. Natidanides Chlamydoselachius Heptanchus |
| Silurian |
17. Ganoids Plated-fishes Proganoids | 17. Accipenserides (Sturgeons) Polypterus |
| Devonian | 18. Dipneusta Paladipneusta | 18. Neodipneusta Ceratodus Proptopterus |
| Carboniferous | 19. Amphibia Stegocephala | 19. Phanerobranchia Salamandrina (Proteus, triton) |
| Permian | 20. Reptilia Proreptilia | 20. Rhynchocephalia Primitive lizards Hatteria |
| Triassic | 21. Monotrema Promammalia | 21. Ornithodelphia Echidna Ornithorhyncus |
| Jurassic | 22. Marsupialia Prodidelphia | 22. Didelphia Didelphys Perameles |
| Cretaceous | 23. Mallotheria Prochoriata | 23. Insectivora Erinaceida (Ictopsia +) |
| Older Eocene | 24. Lemuravida Older lemurs Dentition 3. 1. 4. 3. | 24. Pachylemures (Hyopsodus +) (Adapis +) |
| Neo-Eocene |
25. Lemurogona Later lemurs Dentition 2. 1. 4. 3. | 25. Autolemures Eulemur Stenops |
| Oligocene |
26. Dysmopitheca Western apes Dentition 2. 1. 3. 3. | 26. Platyrrhinæ (Anthropops +) (Homunculus +) |
| Older Miocene | 27. Cynopitheca Dog-faced apes (tailed) | 27. Papiomorpha Cynocephalus |
| Neo-Miocene | 28. Anthropoides Man-like apes (tail-less) | 28. Hylobatida Hylobates Satyrus |
| Pliocene | 29. Pithecanthropi Ape-men (alali, speechless) | 29. Anthropitheca Chimpanzee Gorilla |
| Pleistocene | 30. Homines Men with speech | 30. Weddahs Australian negroes |
Chapter XXI.
OUR FISH-LIKE ANCESTORS
Our task of detecting the extinct ancestors of our race among the vast numbers of animals known to us encounters very different difficulties in the various sections of man’s stem-history. These were very great in the series of our invertebrate ancestors; they are much slighter in the subsequent series of our vertebrate ancestors. Within the vertebrate stem there is, as we have already seen, so complete an agreement in structure and embryology that it is impossible to doubt their phylogenetic unity. In this case the evidence is much clearer and more abundant.
The characteristics that distinguish the Vertebrates as a whole from the Invertebrates have already been discussed in our description of the hypothetical Primitive Vertebrate (Chapter XI, Figs. 98–102). The chief of these are: (1) The evolution of the primitive brain into a dorsal medullary tube; (2) the formation of the chorda between the medullary tube and the gut; (3) the division of the gut into branchial (gill) and hepatic (liver) gut; and (4) the internal articulation or metamerism. The first three features are shared by the Vertebrates with the ascidia-larvæ and the Prochordonia; the fourth is peculiar to them. Thus the chief advantage in organisation by which the earliest Vertebrates took precedence of the unsegmented Chordonia consisted in the development of internal segmentation.
The whole vertebrate stem divides first into the two chief sections of Acrania and Craniota. The Amphioxus is the only surviving representative of the older and lower section, the Acrania (“skull-less”). All the other vertebrates belong to the second division, the Craniota (“skull-animals”). The Craniota descend directly from the Acrania, and these from the primitive Chordonia. The exhaustive study that we made of the comparative anatomy and ontogeny of the Ascidia and the Amphioxus has proved these relations for us. (See Chapters XVI and XVII.) The Amphioxus, the lowest Vertebrate, and the Ascidia, the nearest related Invertebrate, descend from a common extinct stem-form, the Chordæa; and this must have had, substantially, the organisation of the chordula.
However, the Amphioxus is important not merely because it fills the deep gulf between the Invertebrates and Vertebrates, but also because it shows us to-day the typical vertebrate in all its simplicity. We owe to it the most important data that we proceed on in reconstructing the gradual historical development of the whole stem. All the Craniota descend from a common stem-form, and this was substantially identical in structure with the Amphioxus. This stem-form, the Primitive Vertebrate (Prospondylus, Figs. 98–102), had the characteristics of the vertebrate as such, but not the important features that distinguish the Craniota from the Acrania. Though the Amphioxus has many peculiarities of structure and has much degenerated, and though it cannot be regarded as an unchanged descendant of the Primitive Vertebrate, it must have inherited from it the specific characters we enumerated above. We may not say that “Amphioxus is the ancestor of the Vertebrates”; but we can say: “Amphioxus is the nearest relation to the ancestor of all the animals we know.” Both belong to the same small family, or lowest class of the Vertebrates, that we call the Acrania. In our genealogical tree this group forms the twelfth stage, or the first stage among the vertebrate ancestors (p. 228). From this group of Acrania both the Amphioxus and the Craniota were evolved.
The vast division of the Craniota embraces all the Vertebrates known to us, with the exception of the Amphioxus. All of them have a head clearly differentiated from the trunk, and a skull enclosing a brain. The head has also three pairs of higher sense-organs (nose, eyes, and ears). The brain is very rudimentary at first, a mere bulbous enlargement of the fore end of the medullary tube. But it is soon divided by a number of transverse constrictions into, first three, then five successive cerebral vesicles. In this formation of the head, skull, and brain, with further development of the higher sense-organs, we have the advance that the Craniota made beyond their skull-less ancestors. Other organs also attained a higher development; they acquired a compact centralised heart with valves and a more advanced liver and kidneys, and made progress in other important respects.
Fig. 247—The large marine lamprey (Petromyzon marinus), much reduced. Behind the eye there is a row of seven gill-clefts visible on the left, in front the round suctorial mouth.