loo BRAINS OF RATS AND MEN 



a terminal branch of the vertebrate genealogical tree. 

 They are in some respects more highly specialized 

 than any mammals, but this differentiation has taken 

 a different direction. Their reptilian ancestors having 

 attained a highly elaborated organization of innate 

 or instinctive behavior with corresponding develop- 

 ment of subcortical cerebral structure, they have not 

 deviated from the trend of evolution thus early laid 

 down. In accordance with the principles of irreversi- 

 ble differentiation or natural orthogenesis which I 

 have elsewhere explained (1920), they have not in 

 any case dedifferentiated their complicated tissues 

 sufficiently to start a new direction of specialization 

 such as we find in mammahan cortical evolution. 



REPTILES 



We know from paleontology that the ancestors 

 of mammals were primitive reptiles or reptile-like 

 animals whose general organization was more like 

 that of recent turtles (except for the shell) than of any 

 other American animal species. And the endocranial 

 casts of early mammalian species (e.g., Dinoceras) 

 are said by Elliot Smith (1910) to be "curiously 

 reptihan-like.*' In the turtles the relations of the 

 emergent cerebral cortex to underlying parts of the 

 brain are clearly revealed. 



Without going into the details of internal struc- 

 ture of these parts, it is evident that the thickening 

 of the lateral wall of the hemisphere is correlated with 



