2 56 DIVERSIONS OF A NATURALIST 



and maturity of structure justify such a delay is not 

 obvious. The history of our schools and universities 

 and the records as to the age at which marriage takes 

 place bear evidence of this modern increase of the 

 duration of adolescence. In any case, whether the 

 prolongation of the period of physical growth and 

 development is even now still being increased, it is 

 certain that the extension has taken place in former 

 ages, and that the mental development of man is directly 

 related in the first place to this increased period of 

 growth, and in the second place to the prolongation of 

 the period of organized " education " directed by the 

 elder generation. The brain of the human child at 

 four years of age may not infrequently reach as much 

 as 1300 units in volume — more than double that of a 

 full-grown gorilla — and it continues to increase in volume 

 for some eight years, though it is difficult to say precisely 

 when the interlocking of the bony pieces of the skull 

 reaches a point when they can no longer yield to the 

 expansion of the brain. The increase of the cavity of 

 the skull practically ceases in childhood, and the increase 

 in the size of the head subsequently is due to the in- 

 creased size of muscles and fibrous structures on the 

 outer surface of the brain-box. True as it is that man's 

 brain is much larger than that of the higher apes, it is 

 also true that the difference is far greater between the 

 higher apes and the lower monkeys both as to the size 

 of the brain and the complexity of the folds and furrows 

 which mark the surface of the cerebral hemispheres. In 

 these respects, as in every other anatomical feature, as 

 was insisted by Huxley, there is less difference between 

 man and the higher apes than between the higher apes 

 and the lower monkeys, so that there is no pretext for 

 placing man in a group apart from the apes and monkeys 

 or for suggesting the existence of any great structural 



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