THE RECAPITULATION THEORY 



177 



At a later stage most of the gill-pouches have been obliterated, but one 

 is retained and converted into the Eustachian canal, which connects 

 the throat with the middle ear, inside of the ear-drum. Similarly, the 

 embryological evidence shows that the lungs of air-breathers have been 

 derived from the swim-bladder of fishes, a conclusion which had 

 already been reached by comparative anatomy, for in a remarkable 



A B ^ 



Fig. 39. — Embryos in corresponding stage of development of shark (A), 

 fowl (B), and man (C); g, gill slits. {From Scott.) 



group, known as the Dipnoi or lung-fishes, the air-bladder is utilized 

 for purposes of respiration. 



It has been objected that, while embryology may prove relation- 

 ship within a single type, it fails to demonstrate any connection 

 between different types, but this is not altogether true. The Tuni- 

 cata, a curious group of marine animals once referred to the Mollusca, 

 are shown by their ontogeny to be related to the vertebrates and the 

 same is true of certain marine worms {Balanoglossus). Indeed, most 

 modern zoologists have adopted a scheme of classification, in which 



