INDIVIDUAL AND PAL^EONTOLOGICAL EVOLUTION 263 



teeth in the adult stage, there exist, at first, two 

 pairs of oblique, symmetrical teeth, which recall 

 the teeth of the Spondyls and of the Plicatules, 

 and point to ancestral affinities with the types of the 

 Isodont group. Thus the study of the embryonic 

 stages has already permitted, and no doubt will 

 still further permit, the real phylogenic relations 

 between the different families of the Lamelli- 

 branchs to be established. 



As regards Vertebrates, the study of individual 

 development is more restricted in its application, 

 for the reason that embryonic or even young 

 subjects are not generally preserved in the fossil 

 state, and that the progress of ossification naturally 

 causes the disappearance in the adult stage of those 

 phases through which the embryo and the young indi- 

 vidual have passed. There can be quoted, as an ex- 

 ceptional example, the milk teeth of the Mammals, 

 which are frequently enough preserved in a fossil 

 state, and furnish very interesting indications, 

 but of which the interpretation is somewhat difficult. 

 Many opinions have, in fact, been formulated on 

 the signification of milk teeth in relation to the 

 definitive dentition. Some have seen in them a 

 sort of reminiscence of ancestral dental structure. 

 According to Riitimeyer, the milk teeth of the 

 Ungulates not infrequently preserve characteristics 

 belonging to forms geologically and genetically 

 earlier, which are no longer present in the definitive 

 dentition. We ought thus to have a means of 

 securing a retrospective view of the genealogy of 

 each group. Other palaeontologists have seen, on 

 the contrary, in the structure of the milk teeth a 



