186 THE RATE OF GROWTH [ch. 



We see that neither brain, heart nor Hver keeps pace by any 

 means with the growing weight of the whole; there must then 

 be other parts of the fabric, probably the muscles and the bones, 

 which increase more rapidly than the general average. Heart and 

 liver grow nearly at the same rate, the liver keeping a little ahead 

 to begin with, and the heart making up on it in the end; by the 

 age of twenty-five both have multiplied their original weight at 

 birth about thirteen times, but the body as a whole has multiplied 

 by twenty-one. In contrast to these the brain has only multiplied 

 its weight about three and three-quarter times, and shews but little 

 increase since the child was four or five, and hardly any since it 

 was eight years old. Man and the gorilla are born with brains much 

 of a size; but the gorilla's brain stops growing very soon indeed, 

 while the child's has four years of steady increase. The child's 

 brain grows quicker than the gorilla's, but the great ape's body 

 grows much quicker than the child's; at four years old the young 

 gorilla has reached about 80 per cent, of his bodily stature, and the 

 child's brain has reached about 80 per cent, of its full size. 



Even during foetal life, as well as afterwards, the relative weight of the 

 brain keeps on declining. It is about 18 per cent, of the body- weight in the 

 third month, 16 per cent, in the fourth, 14 per cent, in the fifth; and the 

 ratio falls slowly till it comes to about 12 per cent, at birth, say 10 per cent, 

 a year afterwards, and little more than 2 per cent, at twenty*. Many statistics 

 indicate a further decrease of brain-weight, actual as well as relative. The 

 fact has been doubted and denied ; but Raymond Pearl has shewn evidence 

 of a slow decline continuing throughout adult life f. 



The latter part of the table shews the decreasing weights of the 

 organs compared with the body as a whole: brain, which was 

 12 per cent, of the body- weight at birth, falling to 2 per cent, at 

 five-and-twenty ; heart from 0-76 to 0-46 per cent.; liver from 

 4-6 to 2-78 per cent. The thyroid gland (as we know it in the rat) 

 grows for a few weeks, and then diminishes during all the rest of 

 the creature's Hfetime; even during the brief period of its own 

 growth it is growing slower than the body as a whole. 



It is plain, then, that there is no simple and direct relation, holding 



*■ Cf. J. Ariens Kappers, Proc. K. Akad. Wetensch., Amsterdam, xxxix, No. 7, 1936. 

 t R. Pearl, Variation and correlation in brain-weight, Biometrika, iv, pp. 13-104, 

 1905. 



