198 ARBOREAL :\L\N 



metatherians. Anatomically, this question receives an 

 almost perfect answer. Without entering into a bcAvilder- 

 ing array of interesting anatomical details which, deter- 

 mined by Owen, were somewhat obscured by later 

 writers, only to be defined with more striking emphasis 

 by Elliot Smith and other recent workers, we may assume 

 that, on the whole, it was the development of the corpus 

 callosum and all its associated structures that gave the 

 eutherian brain its psychical as well as its anatomical 

 distinction (see Figs. 72 and 73). A true corpus callosum 

 — the great cross-connecting bond of the two neopallial 

 areas — is the outstanding feature of the eutherian brain, 

 and is the index of its neopallial j^erfection. Without 

 neopallial possibilities, educational advantages and physi- 

 cal perfections come in vain to the animal. 



The evolution of the free and mobile fore-limb in 

 arboreal life may be likened to the production of a musical 

 instrument — an insti-ument upon which it is impossible 

 for the animal to produce a full range of harmony, or to 

 appreciate the psychical connotations of this harmony, 

 unless adequate cerebration is developed coincident ly. 



Once again in the evolutionary story we are forced 

 back to consider a combination of seemingly trivial, and 

 apparently chance, associations; in this case the dawning 

 possibilities of neopallial developments combined with 

 the physical adaptations due directly to environmental 

 influences. 



Some authorities have ascribed great, and possibly 

 undue, influence to the changes in brain architecture, 

 while some have concentrated upon the purely bodily 

 adaptations. The solution of the problem lies probably 

 in the consideration of the mean of these two influences. 

 Physical perfections of adaptation are useless, unless 

 advantage can be taken of them by a specialized type of 

 brain; but specialization of the cerebral architecture 

 cannot proceed in the absence of, yet cannot create, 

 physical specializations in evolution. The earliest 



