THE BRAIN AND THE BODY 197 



These arboreal Metatherians have had all the educational 

 advantages of a thoroughly arboreal life; nothing that 

 we have pictured has failed to exert its influences upon 

 them, and yet it is obvious that the advantage that they 

 have taken of it has been slight. There are metatherian 

 convergent mimics of Carnivora, Rodentia, Insectivora, 

 and of most other Eutherian orders, but there is no 

 metatherian convergent mimic of the eutherian Primates. ; 

 It would not be unnatural, therefore, to assume that the 

 full advantage could not be grasped by the metatherian 

 animals, since the ground-plan of their brain would not 

 permit it. Climbing metatherians with perfectly mobile 

 fore-limbs and grasping members were at one time classed, 

 upon the strength of this feature, amongst the Cheiropeds, 

 a group which included only them (Didelphidce, etc.) 

 and the Primates; but they were sorry companions for 

 the Monkeys and the Lemurs in all other respects. Life 

 habit has made them physical mimics, in some degree, 

 of the Eutherian Primates ; it has not made them mimics 

 in any cerebral feature. Rotating forearms, grasping 

 fingers, opposable thumbs all these features are found 

 in perfect combination in the arboreal metatherians, and 

 yet far short of a human, no anthropoid, no simian, and 

 no lemurine evolution is seen in the Metatheria. Obvi- 

 ously, it is not the bodily adaptations alone that have 

 sufficed to create the possibilities of Primate brain develop- 

 ment. We have followed the changes in physical ad- 

 vances and ssen how these have affected Primate evolution, 

 each physical adaptation leading to new possibilities of 

 cerebral advance. All these physical changes could be 

 followed equally well in the Metatheria, but we should 

 fail to note a corresponding advance in cerebral perfec- 

 tion. It is, therefore, natural to ask if there is any gross 

 condition of brain architecture which will serve to dis- 

 tinguish the metatherian from the eutherian brain, and 

 if this distinction will in any way account for the very 

 slight evolutionary advances made by thoroughly arboreal 



