CHAPTER XX\aiI 

 THE BRAIN AND THE BODY 



We have seen that arboreal life may be regarded as 

 offering opportunities for educational possibilities un- 

 known in terrestrial life. W'c have also seen that it 

 probably brings about certain bodily modifications. We 

 are now confronted by a problem: Did the cerebral 

 advance create the physical adaptations, or did the 

 physical adaptations make possible a cerebral advance ? 

 It would seem, at first sight, that upon such a problem 

 the argument might be as long sustained, and as futile, 

 as that expended upon the question of the priority of the 

 hen or the egg. And yet the question is a very interesting 

 one, and one well \\orthy of attention. It is certainly 

 not to be dismissed by a series of confident and epigram- 

 matic assertions. It is possible that at least a partial 

 solution can be given. 



Using a form of words wellnigh meaningless, but never- 

 theless well understood, we ma}^ say that Nature has 

 made several experiments in brain-building. Vertebrate 

 brains are not built all upon one plan; even within the 

 limits of the [Mammals, brain architecture varies con- 

 siderably in basal design in the Prototheria (Monotremes), 

 Metatheria (Marsupials), and Eutheria (higher Mammals). 

 There is no living prototherian animal which has adopted 

 the arboreal habit, and the few existing members of the 

 Monotremes lead lives of particularly restricted possi- 

 bilities. But many of the Metatheria lead lives as truly 

 arboreal as that of any animal, and, indeed, the Marsupial 

 stock is regarded by some as being 2^1'iniarily arboreal. 



196 



