EVOLUTION OF THE VERTEBRATA. 319 



and of increased calorific capacity. In other words, the circula- 

 tion passes from the cold to the hot blooded tyjDc coincidentally 

 with the changes of structure above enumerated. The accession 

 of a capacity to maintain a fixed temperature while that of the 

 surrounding medium changes, is an important advance in animal 

 economy. 



The brain and nervous system also display a general progress- 

 ive ascent. Leaving the brainless Leptocardii, the Marsipo- 

 branchs and fishes present us with small hemispheres, larger 

 optic lobes, and well-developed cerebellum. The hemispheres 

 are really larger than they appear to be, as Rabl Eiickard has 

 shown * that the supposed hemispheres are only corpora striata. 

 But the superior walls are membranous, and support on their in- 

 ternal side only a layer of epithelial cells, as in the embryos of 

 other Vertebrata, instead of the gray substance. So that, al- 

 though we find that the cerebellum is really smaller in the 

 Batrachia and most Eeptilia than in the fishes, the better de- 

 velopment of the hemispheres in the former gives them the pre- 

 eminence. The Elasmobranchii show themselves superior to 

 many of the fishes in the large size of their corpora restiformia 

 and cerebellum. The Eeptilia constitute an advance on the Ba- 

 trachia. In the latter the optic thalami are, with some excep- 

 tions, of greater diameter than the hemispheres, while the reverse 

 is generally true of the reptiles. The crocodiles display much 

 superiority over the other reptiles in the larger cerebellum, with 

 rudimental lateral lobes. The great development of the hemi- 

 spheres in birds is well known, while the general superiority of 

 the brain of the living Mammalia over all other vertebrates is 

 admitted. 



The consideration of the successive relations of the skeleton in 

 the classes of vertebrates embraces, of course, only the characters 

 which distinguish those classes. These are not numerous. They 

 embrace the structure of the axis of the skull ; of the ear-bones ; 

 of the suspensors of the lower jaw ; of the scapular arch and 

 anterior limb, and of the pelvic arch and posterior limb. Other 

 characters are numerous, but do not enter into consideration at 

 this time. 



The persistence of the primitive cartilage in any part of the 

 skeleton is, embryologically speaking, a mark of inferiority. 



* " Biologischcs Centralblatt," 1884, p. 449. 



