82 THE RATE OF GROWTH [ch. 



A minor, but very curious point brought out by the same investigators 

 is that, if instead of stature we deal with lieight in the sitting posture (or, 

 practically speaking, with length of trunk or back), then the correlations 

 between this height and its annual increment are throughout negative. In 

 other words, there would seem to be a general tendency for the long trunks 

 to grow slowly throughout the whole period under investigation. It is a 

 well-known anatomical fact that tallness is in the main due not to length of 

 body but to length of limb. 



The whole phenomenon of variabiUty in regard to magnitude 

 and to rate of increment is in the highest degree suggesti.ve : 

 inasmuch as it helps further to remind and to impress upon us 

 that specific rate of growth is the real physiological factor which 

 we want to get at, of which specific magnitude, dimensions and 

 form, and all the variations of these, are merely the concrete and 

 visible resultant. But the problems of variabihty, though they 

 are intimately related to the general problem of growth, carry us 

 very soon beyond our present limitations. 



Rate of grow ih in other organisms'^. 



Just as the human curve of growth has its shght but well- 

 marked interruptions, or variations in rate, coinciding with such 

 epochs as birth and puberty, so is it with other animals, and this 

 phenomenon is particularly striking in the case of animals which 

 undergo a regular metamorphosis. 



In the accompanying curve of growth in weight of the mouse 

 (Fig. 12), based on W. Ostwald's observations f, we see a distinct 

 slackening of the rate when the mouse is about a fortnight old, 

 at which period it opens its eyes and very soon afterwards is 

 weaned. At about six weeks old there is ( nother well-marked 

 retardation of growth, following on a very rapid period, and 

 coinciding with the epoch of puberty. 



* See, for an admirable resume of facts, Wolfgang Ostwald, Ueber die ZeitUche 

 Eigenschaften der Entwickelungsvorgdnge (71 pp.), Leip/.ig, 1908 (Roux's Vortrdge, 

 Heft v) : to which work I am much indebted. A long list of observations on the 

 growth-rate of various animals is also given by H. Przibram, Exp. Zoologie, 1913, 

 pt IV ( Vitalitdt), pp. 85-87. 



f Cf . St Loup, Vitesse de croissanoe chez les Souris, Bull. Soc. Zool. Fr. xvm, 

 242, 1893; Robertson, Arch. f. Entwickelung-smech. xxv, p. 587, 1908; Donaldson. 

 Boas Memorial Volume, New York, 1906. 



