Ontogeny Recapitulates Phylogeny 83 



replaced by its substitute, the backbone. This develops 

 from the mesodermic sheath of the notochord, and 

 may perhaps utilize the notochord as a sort of scaf- 

 folding during the time of formation. Among the 

 different types of fishes we can find every stage in 

 the replacement of the endodermic notochord by the 

 mesodermic backbone; but in Amphibians, Reptiles, 

 Birds, and Mammals the notochord is an embryonic 

 structure with only vestiges left after development 

 has been completed. The remarkable fact is that on 

 to man himself the notochord continues to appear in 

 development, though it never comes to anything 

 above the level of fishes. It is a veritable antique; it 

 has its short day and passes. But it is never absent, 

 and this would be unmeaning if we had not the evolu- 

 tionist interpretation. 



In the individual's building up of important struc- 

 tures such as the brain, the skull, the heart, the kid- 

 nevs, the foundations are laid on old-fashioned lines, 

 often not directly suggestive of what is to follow. 

 The individual development recapitulates in modi- 

 fied form the racial evolution. There is always 

 specificity; each embryo is itself and no other; yet 

 the old paths are to some extent re-trod, even if it 

 means circuitousness. Often, however, the progress 

 of a hundred thousand years is condensed into one 

 day. 



The bony flat-fishes, such as plaice, flounder, sole, 

 and turbot, are very peculiar in many ways. They 

 rest and swim on one side, on the floor of the sea. 

 Plaice, soles, lemon soles, dabs, flounders, witches, 



