THE HISTOEY OF OUK SPECIES. 85 



correct. For the historical succession of vertebrate 

 fossils corresponds completely with the morphological 

 evolutionary scale which is revealed to us by com- 

 parative anatomy and ontogeny. After the Silurian 

 fishes come the dipnoi of the Devonian period — the 

 Carboniferous amphibia, the Permian reptilia, and the 

 Mesozoic mammals. Of these, again, the lowest forms, 

 the monotremes, appear first in the Triassic period, 

 the marsupials in the Jurassic, and then the oldest 

 placentals in the Cretaceous. Of the placentals, in 

 turn, the first to appear in the oldest Tertiary period 

 (the Eocene) are the lowest primates, the prosimire, 

 which are followed by the simia3 in the Miocene. Of 

 the catarrhinse, the cynopitheci precede the anthro- 

 pomorpha ; from one branch of the latter, during the 

 Pliocene period, arises the ape-man without speech 

 (the pithecanthropus alalus) ; and from him descends, 

 finally, speaking man. 



The chain of our earlier invertebrate ancestors is 

 much more difficult to investigate and much less safe 

 than this tree of our vertebrate predecessors ; we have 

 no fossilised relics of their soft, boneless structures, 

 so palaeontology can give us no assistance in this case. 

 The evidence of comparative anatomy and ontogeny, 

 therefore, becomes all the more important. Since the 

 human embryo passes through the same chorchda- 

 stage as the germs of all other vertebrates, since it 

 evolves, similarly, out of two germinal layers of a 

 gastrula, we infer, in virtue of the biogenetic law, the 

 early existence of corresponding ancestral forms — 

 vermalia, gastrseada, etc. Most important of all is 

 the fact that the human embryo, like that of all other 

 animals, arises originally from a single cell ; for this 

 "stem-cell" (cytida) — the impregnated egg-cell — points 



