90 



THE RATE OF GROWTH 



[CH. 



age of twenty-five they have multiphed their weight at birth by 

 about thirteen times, while the weight of the entire body has been 

 multiphed by about twenty-one ; but the weight of the brain has 

 meanwhile been multiphed only about three and a quarter times. 

 In the next place, we see the very remarkable phenomenon that 

 the brain, growing rapidly till the child is about four years old, then 

 grows more much slowly till about eight or nine years old, and 

 after that time there is scarcely any further perceptible increase. 

 These phenomena are diagrammatically illustrated in Fig. 18. 



5 10 15 20 ^ears 25 



Fig. 18. Relative growth in weight (in Man) of Brain, Heart, and 

 whole Body. 



Many statistics indicate a decrease of brain-weight during adult life. 

 Boas* was inclined to attribute this apparent phenomenon to our statistical 

 methods, and to hold that it could " hardly be explained in any other way 

 than by assuming an increased death-rate among men with very large brains,, 

 at an age of about twenty years." But Raymond Pearl has shewn that there 

 is evidence of a steady and very gradual decline in the weight of the brain 

 with advancing age, beginning at or before the twentieth year, and con- 

 tinuing throughout adult lifef. 



* I.e. p. 1542. 



t Variation and Correlation in Brain-weight, Biometrika, iv, pp. 13-104, 1905. 



