and Embryological Development. 349 



uniting Birds and Reptiles on the one side and Batrachians 

 and Fishes on the other. It is to embryology that we owe 

 the explanation of the affinities of the old Fishes in which 

 Agassiz first recognized the similarity to the embryo of 

 Fishes now living, and by its aid we may hope to understand 

 the relationship of the oldest representatives of the class. It 

 has given us the only explanation of the early appearance 

 of the Cartilaginous Fishes, and of the probable formation 

 of the earliest vertebrate limb from the lateral embryonic 

 fold, still to be traced in the young of the Osseous Fishes of 

 to-day. 



Embryology has helped us to understand the changes 

 aquatic animals must gradually undergo in order to become 

 capable of living upon dry land. It has given us pictures 

 of swimming-bladders existing as rudimentary lungs in 

 Fishes with a branchial system ; in Batrachians it has shown 

 us the persistence of a branchial system side by side with 

 a veritable lung. We find among the earliest terrestrial 

 Vertebrates types having manifest , affinities with the Fishes 

 on one side and Batrachians on the other, and we call these 

 types Reptiles ; but we should nevertheless do so with a 

 reservation, looking to embryology for the true meaning of 

 these half-fledged Reptiles, which lived at the period of 

 transition between an aquatic and terrestrial life, and must 

 therefore always retain an unusual importance in the study 

 of the development of animal life. 



When we come to the embryology of the marine Inverte- 

 brates, the history of the development of the barnacles is too 

 familiar to be dwelt upon ; and I need only allude to the 

 well-known transformations of the Echinoderms, of the Aca- 

 lephs, Polyps, in fact of every single class of Invertebrates, 

 and perhaps in none more than in the Brachiopods, to show 

 how far-reaching has been the influence of embryology in 

 guiding us to a correct reading of the relations between 

 the fossils of successive formations. There is scarcely an 

 embryological monograph now published dealing with any 

 of the later stages of growth which does not speak of their 

 resemblance to some type of the group long ago extinct. 

 It has therefore been most natural to combine with the 

 attempts constantly made to establish the genetic sequence 

 between the genera of successive formations an effort to 

 establish also a correspondence between their palasontological 

 sequence and that of the embryonic stages of development of 

 the same, thus extending the mere similarity first observed 

 between certain stages to a far broader generalization. 



It would cany me too far to sketch out, except in a most 



