VERTEBRATE ANCESTORS OF MAN ? 6 1 



Silurian period, as is indicated by the fossil remains 

 of teeth and fin spines. 



Our twelfth ancestral stage, according to Haeckel, 

 was represented by animals which probably possessed 

 a distant resemblance to the still living lepido-siren 

 tribe of fishes. They were evolved from proto-fishes 

 by metamorphosis of the swimming bladder into a 

 lung, and conversion of the nasal cavities into air- 

 passages opening into the mouth. 



With this stage began the series of man's ances- 

 tors breathing by lungs. A branch of Proto-fishes 

 or Selachii during the Devonian period, Haeckel 

 thinks, made the first successful attempt — the first 

 successful attempt, to repeat the expression — to live 

 on land and breathe air ; whereby the swimming- 

 bladder became a lung with a corresponding change 

 in the structure of the heart. 



From the amphibious fishes of the twelfth stage 

 just described, the pcrenni-branchiate batrac/iians, 

 forming the thirteenth stage, were evolved. The 

 extinct unknown form from which man descended, 

 Haeckel designates by the name of Amphibium. It 

 retained its gills throughout life like the still living 

 proteus and axolotl ; and its evolution from the 

 amphibious lepido-siren took place by metamorphosis 

 of the paddle-like fins into legs with five-toed feet, 

 and by a higher ( differentiation ' of various organs, 

 especially the vertebral column. This perenni- 

 branchiate amphibium existed about the middle of 

 the Palaeolithic or Primary time — perhaps before 



