EMBEYOGENIC METHOD 51 



an hypothetical ancestor common to the Tunicates 

 and to the Vertebrates. 



From the Acranian Vertebrates may have issued, 

 on the one hand, the class of Cyclostoms or lam- 

 preys having a circular mouth serving as sucker ; 

 on the other that of fish possessing two jaws and 

 two pairs of limbs. The group of Selachians, com- 

 prising the two types of Dog fish and Skates with 

 cartilaginous frame, may be the most primitive of 

 all and have produced, by bifurcation, on the one 

 hand, the Ganoid and Bony Fishes ; on the other, the 

 Amphibians, by passing through the Dipneusta 

 with duplicate respiration at once branchial and 

 pulmonary. From the group of Fishes may also de- 

 scend the curious and important group of the 

 swimming Halisaurians, comprising the genera 

 Icthyosaurus and Plesiosaurus, which peopled the 

 seas in the Triassic, Jurassic, and Cretacean epochs. 



The Amphibians possessing, in the adult, aerian 

 respiration and pentadactyl limbs gave birth to 

 the higher Vertebrates, characterized by having 

 their embryos enveloped in an amnion or mem- 

 brane. This evolution must have been produced 

 by two diverging branches, the one terminating in 

 the Reptiles and Birds, the other in the Mammals. 



The whole of this genealogy of the Vertebrates by 

 Haeckel is far from indisputable from a palae- 

 ontological point of view. The Selachians, very 

 common, it is true, in Primary times, are not the 

 earliest Fishes known and are preceded in the lower 

 Silurian by the armoured Ganoids or Placoderms. 

 On the other hand no palaeontologist would sub- 

 scribe without diffidence to the direct linking of the 



