Physical Evolution of Man 571 



In the preceding chapter we have tried to demonstrate, and 

 we trust successfully, that the more complex brain structure, 

 and the higher mental qualities of diverse groups of animals, 

 over others closely allied to them taxonomically, have resulted 

 from an increasing proenvironal use of parts around and ac- 

 cessory to the brain. The stimuli thus sent in increasing num- 

 ber and variety to the brain have stimulated it to increase in 

 its cells and fibers, and these in turn have sent an increased 

 number and variety of efferent stimuli outward. We shall 

 now endeavor to learn how far this principle can be made ap- 

 plicable to man. 



Along the entire mammalian line that leads from the mar- 

 supials to man, we have seen that varied use of the forelimbs, 

 and a rearing of the animal more and more into a semi-erect 

 and ultimately erect posture, have proceeded pari passu with 

 added complexity in brain structure and action response. In 

 man, as in the lower animals, increase in quantity and com- 

 plexity of the brain matter — a purely material though highly 

 complex chemical mass — can only proceed tlu'ough increasing 

 quantity and complexity of the energizing currents that traverse 

 it. 



That the erect posture is an effect and not a cause is clearly 

 proven by study of the mantis, of penguins and many other 

 birds, none of which possesses an exceptional degree of nervous 

 complexity. Again human cerebral increase has evidently 

 not resulted from special stimulating inflows of afferent energy, 

 through the special sense-organs as the following show. 



Had olfactorj^ stimuli, entering and affecting the nasal 

 regions, been an important cause, we should have expected to 

 find, as we do in the skate, the buzzard, the wolf, and the dog, 

 a considerable enlargement in the olfactory lobes, and an in- 

 crease in the cerebral tissue associated with these lobes. Such, 

 however, is not the case. 



Again, had heliotropic or light stimuli been of special stimulat- 

 ing value, we should have expected to find the optic lobes in 

 man smaller than in birds and many mammals whose power of 

 vision at the present day greatly excels that of man. But 



