OUR ANCIENT RELATIVES 



the present time, including the famous lung-fish 

 (Neoceratodus) of Australia, all have very well- 

 developed and functional lungs in addition to gills. 

 Moreover, the embryonic development of the mod- 

 ern lung-fish, it has been shown, closely parallels 

 that of certain existing salamanders. 



Nevertheless, all the fossil and recent fishes of 

 this dipnoan group had definitely and hopelessly 

 removed themselves from the main line of ascent, 

 since they had already either reduced or lost the 

 marginal bones of the upper jaw and had developed 

 peculiar and specialized fan-shaped cutting plates 

 on the roof of the mouth and on the inner side of 

 the lower jaw. 



The earliest of the land-living or four-footed 

 vertebrates, on the contrary, retained the marginal 

 jaw bones and never developed the fan-shaped 

 cutting plates on the roof of the mouth. 



To make a long story short, the real ancestors 



of the higher vertebrates were probably neither 



true dipnoans, nor any of the Devonian lobe-finned 



ganoids, but were the still undiscovered common 



ancestors of these rather closely related groups 



living somewhere, perhaps in Lower Devonian or 



Upper Silurian times. 



25 



