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 



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

 fowl (5), 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 



