92 



THE RATE OF GROWTH 



[CH. 



These equations are applicable to ages between fifteen and eighty ; 

 if we take narrower limits, say between fifteen and fifty, we can get 

 a closer agreement by using somewhat altered constants. In the 

 two sexes, and in different races, these empirical constants will be 

 greatly changed* . Donaldson has further shewn that the correla- 

 tion between brain-weight and body-weight is very much closer 

 in the rat than in man"]". 



The falling ratio of weight of brain to body with increase of size or age 

 finds its parallel in comparative anatomy, in the general law that the larger 

 the animal the less is the relative weight of the brain. 



For much information on this subject, see Dubois, " Abhangigkeit des 

 Hirngewichtes von der Korpergrosse bei denSsiUgethieren,''' Arch. f. Anthropol. 

 XXV, 1897. Dubois has attempted, but I think with very doubtful success, 

 to equate the weight of the brain with that of the animal. We may do this, 

 in a very simple way, by representing the weight of the body as a power of 

 that of the brain ; thus, in the above table of the weights of brain and body 

 in four species of cat, if we call W the weight of the body (in grammes), and 

 w the weight of the brain, then if in all four cases we express the ratio by 

 W — w", we find that n is almost constant, and differs little from 2-24 in all 

 four species: the values being respectively, in the order of the table 2-36, 

 2-24, 2-18, and 2-17. But this evidently amounts to no more than an 

 empirical rule; for we can easily see that it depends on the particular scale 

 which we have used, and that if the weights had been taken, for instance, 

 in kilogrammes or in milligrammes, the agreement or coincidence would not 

 have occurred J. 



* Biometrika, iv, pp. 13-104, 1904. 



t Donaldson, H. H., A Comparison of the White Rat with Man in respect to 

 the Growth of the entire Body, Boas Memorial Vol., New York, 1906, pp. 5-26. 



t Besides many papers quoted by Dubois on the growth and weight of the 

 brain, and numerous papers in Biometrika, see also the following: Ziehen, Th., 

 Das Gehirn : Massverhaltnisse, in Bardeleben's Handh. der Anat. des Menschen, 

 IV, pp. 353-386, 1899. Spitzka, E. A., Brain-weight of Animals with special 

 reference to the Weight of the Brain in the Macaque Monkey, J. Comp. Neurol. 



