94 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



an aerial respiration ; and, second, an increase in the 

 power to aerate and distribute a circulating fluid of in- 

 creased quantit}^ and of increased calorific capacity. 

 In other words, the circulation passes from the cold to 

 the hot-blooded type coincidentally with the changes 

 of structure above enumerated. The accession of a ca- 

 pacity to maintain a fixed temperature while that of 

 the surrounding medium changes, is an important ad- 

 vance in animal economy. 



The brain and nervous system also display a gen- 

 eral progressive ascent. Leaving the brainless Acrania, 

 the Marsipobranchs and fishes present us with small 

 hemispheres with thin cortex, larger optic lobes, and 

 well-developed cerebellum. The hemispheres are really 

 larger than they appear to be, as Rabl Riickard has 

 shown ^ that the supposed hemispheres are only corpora 

 striata. But the superior walls are membranous, and 

 support on their internal side only a layer of epithelial 

 cells, as in the embryos of other Vertebrata, instead of 

 the gray substance. So that, although we find that the 

 cerebellum is really smaller in the Batrachia and most 

 Reptilia than in the fishes, the better development of 

 the hemispheres in the former gives them the pre- 

 eminence. The Elasmobranchii show themselves su- 

 perior to many of the fishes in the large size of their 

 corpora restiformia and cerebellum. The Reptilia con- 

 stitute an advance on the Batrachia. In the latter the 

 optic thalami are, with some exceptions, 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 greater 

 development of the hemispheres in birds is well known, 



'iBiologisckes Centralblatt , 1884, p. 449. 



