ioo8 



E\OLLTIONAL MODIFICATIONS 



in which both the fore and hind extremities were employed in a locomotor 

 capacity. Yet in all primates the beginning of this manual expansion is 

 clearly discernible. By means of this differentiation they entered upon a new 



Tabllation of Plaximetric Coefficients 



Comparative planimetric indices of four important structures in the brain stem of vertebrates. These 

 indices show the marked evolutional expansion in all four structures, especially affecting the primates and 

 emphatically demonstrating the effects of arborealism. Three of the structures are exclusively mammalian. 

 The fourth, the inferior olivarj" nucleus, is rudimentary in the bird and, when present, in all other lower 

 vertebrates. 



world and in the organization of their brain plainly reveal that the key to 

 this new realm was already in their possession. 



The development of the paw, the hoof, the claw or the swimming paddle 

 condemned its possessor to the restrictive conditions of a greatly limited 

 habitat. It would actually' seem, on the other hand, as though the inception 

 of manual differentiation were an invitation to new freedom, an inspiration 

 for the exercise of that latent power of supremacy which awaited only the 

 acquisition of some adequate instrument whereby to express itself. 



Even the low organization of the brain in lemur discloses the first signs 

 of the new impulse to expand the neokinetic portion of the central nervous 



