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. Platyrrhinae. (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 2.21. 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 1.11, Figure 1.98 to 1.102). 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-larvae 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 2.16 and 2.17.) The Amphioxus, the lowest Vertebrate, and the Ascidia, the nearest related Invertebrate, descend from a common extinct stem-form, the Chordaea; and this must have had, substantially, the organisation of the chordula.