I024 EVOLUTIONAL MODIFICATIONS 



A comparative survey of the Nciitral surface of the [)rain stem discloses 

 at once progressive extension in the neopallium. The first appearance of the 

 pyramid, the ])ons Varolii and the eerehral ])eduncles announces the acKent 

 of the mammalian orders and mtroduces a new epoch ot bram dexelopment. 

 Along many dillerent hues ol actixity the mammals ha\'e endeavored to make 

 use of their new cerebral endowments. Manx' diflerent kmds ol extreniity 

 differentiation have responded to this cerebral specialization in order to 

 develop new means lor expressing the potentialities ol neokmesis. Hoofs, 

 claws, paws and e\en paddles in the pinniped carnixores ha\c made their 

 appearance as the agents tor externalizing these new caj^acities ot bi'hax lor. 

 Each of thest' In its turn has labored under thost' inherent disad\ antagcs 

 ])eeuliar to appendicular structures w hich are w holl\ dominated by locomotor 

 function. Tlu' specilicitx' ol locomotor pcrlormance in such forms as the 

 kangaroo, the rabbit, tlu' giratle, the ckphant. the horse, the whale or the seal 

 curtailed the opportunities which these animals might possess for the 

 upbiiifding of a highly comj:)le\ sphere ol bcha\ioial reaction. To some extent 

 the diflerent Kit Kin of the extremities was ol st-r\K\' in utilizing (.'iiergy ni 

 combat and in obtaining food supply, liut the motor tornuilas expressed in 

 locomotor acti\ it\ , in acts of ollense and delense, or in ciuest ot lood, with 

 the exception ot their distinct mereast' in the complexit\ ol motor pattern, 

 show little ot real adxancc o\cr such lower xcrtcbrates as the birds and the 

 reptiles, in most ot the mammalian oixK'is the imj^ression is unavoidable that 

 all attempts toward de\ eloping the splu're ol lU'okinesis, howe\er ob\iousIy 

 seeking adecjuatc channels through which to express themselves most etlec- 

 tively, came lor the most part to naught, or only advanced to indecisive 

 advantages, until the factors culminating in the primate order made them- 

 selves felt in a crucial specialization. The fort'- and hmdlinibs, designed pri- 

 marily as locomotor organs, now attained thosi' powers capable at length of 

 throwing ofl the yoke which subjugated the extremities to the ser\ice of 



