EVIDENCES OF EVOLUTION 55 



at least, the path taken by the embryos of 

 its ancestors. 



Let us take the particular case of the 

 notochord, a supporting axial rod, present for 

 some time at least in all Vertebrate embryos, 

 and always arising hi the came way as a fold 

 along the dorsal median line of the embryonic 

 gut. In a few old-fashioned types, such as 

 lancelet and lamprey, there is no backbone, 

 but the notochord persists throughout life. 

 From fishes upwards it is gradually replaced 

 Sn development by the backbone. The noto- 

 chord does not become the backbone, but is 

 replaced by it. The two arc quite different 

 embryologically, the notochord arising from 

 the inner germinal layer or endoderm, the 

 backbone arising, like the rest of the internal 

 skeleton, from the middle germinal layer or 

 mesoderm. In point of fact, the backbone 

 develops from a mesodermic sheath around the 

 notochord, a permanent structure around a 

 temporary structure, as a tall tower might 

 be built around an internal scaffolding of wood. 

 Now, what is the relation between the more 

 primitive axis or aotochord and its more 

 effective substitute the backbone, seeing that 

 the former does not become the latter ? In 

 his interesting theory of " the substitution of 

 organs," Kleinenberg suggested that the noto- 



