192 THE PATH OF DEGENERATIVE EVOLUTION 



of this atrophy does not correspond in any way to 

 the order of the formation of the vessels. 



The median parts of the anterior two lateral 

 branches (m 1 and m 2 ) disappear, and the vertical 

 parts remain as the internal and external carotid 

 vessels. The vertical piece which joined the 

 posterior parts of the third and fourth arches dis- 

 appears : the internal and external carotids thus 

 acquire a stem of their own. The parts of the 

 fourth arch remain ; the fifth arch disappears at 

 each side, and the sixth arch forms the pulmonary 

 artery (ap.}. 



Thus the degeneration of these vessels represents 

 in no way whatever a retracing of their develop- 

 mental history. All that occurs is that the useless 

 parts disappear and the useful parts persist. A 

 comparative study of this example would only 

 enforce our conclusion. 



In ontogeny the neurapophyses are more ancient 

 than the vertebral centres. None the less, as we 

 have already seen, the examination of any vertebral 

 column from head toward tail shows a gradual 

 disappearance of all parts except the centra, 

 although the centra are the last to be formed. 



SECTION II. 

 The path of degeneration in plants. 



1. Rarity of cases of recapitulation in the 

 organogeny of leaves, We have already said that 



