958 EVOLUTIONAL MODIFICATIONS 



differentiation which led to the \arious orders of mammals started from some 

 simple lissencephahc condition, showing only the most primiti\e fissures. 

 It may, therefore, be concluded that the hssural prototype of the j)rimate 

 order is ah'eady hiid down m the h'lnur hram. Such departures as it makes 

 from tlu' uhimate design may be ascribed to specific specializations m tlie 

 U'lnurs themseUes. To whatever extent they experienced the primordial 

 incentive toward primate dillerentiation, there was that in their organization 

 which determined characteristics strictlx sui iicncris, yet imparted to their 

 specialization such tendencies as amply justify their inclusion in a separate 

 suborder of the primates. 



The brain of the marmoset, on the other liaiid, which declares itself in so 

 many features as inherently primate, allords that more generalized pattern 

 in its fissural arrangement from which all of the families in the anthropoidea 

 could take their beginning. The brain of Callithrix jacchus might easily be 

 descendant from some primiti\e lissenccphalic type. It presents but a single 

 well-delined sulcus, the lissure of Sylvius, with the faintest inception of a 

 superior temporal sulcus. Except for these landmarks, the lateral surface 

 of the hemisphere is quite without sign of fissural inscription. W ilh this as 

 yet unprejudiced surface of the neo])allium. inlluenced alone by the correct 

 position of its S\ Kian fissure, the marmoset brain appears to be waiting for 

 the further Impress of those hssural markings characteristic of the more 

 advanced ])rimates. This relatively simple condition of the neopallium in 

 Hapalidae is in harmony w ith what has already been observed in reference 

 to their simjjle behavioral characters. The departure of this family from tlu' 

 primitive primate stock was doubtless a retrogressive one, less significant in 

 what it produced of itself than in its prophetic indications of actual simian 

 tribes to come. In the lemur brain and in the brain of marmoset two contrast- 

 ing tendencies in the early ]jrimate differentiation are represented. In the 

 first instance the lemur, representing the Incipient primate impulses, furnishes 



