PARALLELISM. 195 



aerated blood is distributed to the organs, and all but 

 one of the old roots of the aorta have ceased to function 

 as such. 



This evolutionary succession is preserved with much 

 fidelity in the ontogeny of the respective classes of 

 Vertebrata. The representatives of each class pass 

 through the stages which are permanent in the classes 

 below them in the series. The Mammalia, as the 

 highest class, pass through all the stages. (Fig. 49.) 

 This series coincides also with phylogenetic succes- 

 sion. The order of appearance in time of the Verte- 

 brata is, first Agnatha, then Pisces, Batrachia, Rep- 

 tilia, and Mammalia. 



In all the details of structure the same relation 

 may be observed. Referring to the illustrations of 

 phylogeny and variation of character described in the 

 preceding pages, many of the characters definitive of 

 natural divisions have been observed to appear in the 

 course of the embryonic life of those types which pos- 

 sess them. Those of greater systematic significance 

 appear earlier, and those of less importance in a tax- 

 onomic sense, later. I select some illustrations of this 

 principle. 



I have shown that the primitive type of superior 

 molar in the placental Mammalia is tritubercular, the 

 fourth tubercle being added internally and posteriorly 

 in the later forms. Dr. Taeker has recently observed 

 that in the development of the superior molars in the 

 horse, at an early stage the crown is tritubercular, and 

 that the fourth cusp or hypocone is subsequently added, 

 as in the phylogenetic history. As the horse presents 

 the most complex molar among Mammalia, this sur- 

 vival of the record is interesting. 



In the ArtTodactyla and Edentata which lack su- 



