INDIVIDUAL AND PAL^ONTOLOGICAL EVOLUTION 255 



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one most generally employed, consists in finding in 

 geological strata forms which in the adult state 

 reproduce one of the transitory phases through 

 which the development of an existing animal 

 passes. We give to these fossil forms the name of 

 Persistent Embryonic types. Palaeontology is able 

 to furnish us with a fairly large number of examples 

 of this. Thus the Fishes of the Primary epoch, 

 such as the scaly Ganoids, have a soft vertebral 

 column in a state of embryonic tissue or notochord, 

 as in the embryos of existing Ganoids or Teleos- 

 teans. The ossification of the vertebral column 

 takes place progressively from the Silurian to the 

 middle Jurassic, thus spreading over a very long 

 geological period the stages of the individual 

 development of our present Fishes. In the same 

 way the palaeozoic Amphibians pass, as regards the 

 ossification of the vertebral column, through a 

 series of progressive phases : first the lepospondylian 

 stage, in which the bony tissue forms a simple 

 sheath round the centrum which remains soft and 

 embryonic (Branchiosaurus) ; then the temnos- 

 pondylian, in which several points of ossification 

 develop themselves in the vertebral arcs and 

 apophyses, and give bony segments which remain 

 apart from, and do not adhere to, the centrum 

 (Archegosaurus) ; and, finally, the Stereospondylian 

 stage, in which the vertebra is completely ossified, 

 as in the Triassic Labyrinthodons. These different 

 phases are reproduced in the development of our 

 present Reptiles and Amphibians. In recent and 

 modern Ruminants the bones of the metacarpus 

 and of the metatarsus, separate in the embryo and 



