488 Causes and Course of Organic Evolution 



But if we assume it as safe that batrachians had already 

 attained to urodelan level of organization in the lower car- 

 boniferous age — and the Aistopoda practically assure such a 

 possibility — , also that primitive mammals appeared in the 

 early triassic — as all palseontological evidence tends to show — , 

 then the extended period from the upper carboniferous through 

 the permian woidd represent the amphibio-mammalian and 

 promammalian age. 



During this permian period the higher urodeles would have 

 become thoroughly land animals. So the sparse sensitive 

 hair growths would spread abundantly over the body in part 

 for sense perception, in part for aerial temperature protec- 

 tion; the ear membrane would develop round its cartilage an 

 external pinna for collection of the air waves; the quadrate 

 bone would slip in to form a bone of the ear, and the articular 

 be replaced by the squamosal; the three bones of the lower 

 jaw would become, with increased masticatory work on firmer 

 plants or animals, condensed into one; the originally abundant 

 similar vertebrae of Apoda would become from the changed 

 condition in urodeles condensed from 270 in some iVpoda, 

 through the 100 or more of Siren allies and the 70-50 of higher 

 urodeles, to the average number of about 50 for typical mam- 

 mals. With varied running, climbing, and swimming habits 

 on land and in water, these vertebra? would become from spe- 

 cialized muscular action sharply divided into cervical, dorsal, 

 lumbar, sacral, and caudal; with increasingly rapid observa- 

 tion-motion of the head, increase in the cervical vertebra? 

 and associated muscles would occur; greater and more rapid 

 freedom of movement in the air would result in more perfect 

 innervation between muscles and nerves. Surrounded by a 

 medium that would show more rapid and pronounced stimu- 

 lation changes than did water, the entire brain would increase 

 in proportion. The anterior or olfactory lobes, that from 

 Rotifera and Turbellaria upward seem to represent the first 

 formed and most anterior sense center, would increase rapidly 

 in size through increased action and reaction. All of these 

 environal changes and results would be gathered up in and 



