2 20 THE HISTORY OF CEEATIOISr. 



opposed to all other Vertebrate animals which breathed 

 through gills (Branchiata). In all the Vertebrata already 

 discussed, we found that they either always breathed 

 through gills, or at least did so in early life, as in the 

 case of Frogs and Salamanders. On the other hand, we 

 never meet with a Reptile, Bird, or Mammal which at any 

 period of its existence breathes through gills, and the gill- 

 arches and openings which do exist in the embryos, are, 

 during the course of the ontogeny, changed into entirely 

 different structures, viz., into parts of the jaw-apparatus and 

 the organ of hearing. (Compare above, vol. i. p. 307.) All 

 Amnionate animals have a so-called cochlea in the organ of 

 hearing, and a "round window" corresponding with it. These 

 parts are wanting in the Amnion-less animals; moreover, their 

 skull lies in a straight line with the axis of the vertebral 

 column. In Amniotic animals the base of the skull appears 

 bent in on the abdominal side, so that the head sinks upon 

 the breast. (Plate III. Fig. C, D, G, H.) The organs of tears 

 at the side of the eye also first develop in the Amniota. 



The question now is. When did this important advance 

 take place in the course of the organic history of the earth ? 

 When did the common ancestor of all Amniota develop out 

 of a branch of the Non-amniota, to Avit, out of the brapch of 

 the Amphibia ? 



To this question, the fossil remains of Vertebrata do 

 not give us a very definite, but still they do give an 

 approximate, answer. For with the exception of two 

 lizard-like animals found in the Permian system (the 

 Proterosaurus and Rhopalodon), all the fossil remains of 

 Amniota, as yet known, belong to the secondary, tertiary, 

 and quaternary epochs. With regard to the two Vertebrata 



