216 THE RATE OF GROWTH [ch. 



the forceps expand to their full size; if the .reature be weak or 

 underfed, inflation is incomplete and tho tail-forceps remain small. 

 In either case it is an affair of a few critical moments during the 

 final ecdysis; in ten minutes or less, the chitin has hardened, and 

 shape and size change no- more. Willi Kuhl, who has given us this 

 interesting explanation, suggests that the dimorphism observed by 

 Bateson and by Diakonow is not an essential part of the pheno- 

 menon; he has found it in one instance, but in other and much larger 

 samples he has found all gradations, but only a single,- well-marked 

 unimodal peak*. 



The effect of temperature'f 



The rates of growth which we have hitherto dealt with are mostly 

 based on special investigations, conducted under particular local 

 conditions; for instance, Quetelet's data, so far as we have used 

 them to illustrate the rate of growth in man, are drawn from his 

 study of the Belgian people. But apart from that "fortuitous" 

 individual variation which we have already considered, it is obvious 

 that the normal rate of growth will be found to vary, in man and 

 in other animals, just as the average stature varies, in different 

 locahties and in different "races." This phenomenon is a very 

 complex one, and is doubtless a resultant of many undefined con- 

 tributory causes; but we at least gain something in regard to it 

 when we discover that rate of growth is directly affected by 

 temperature, and doubtless by other physical conditions. Reaumur 

 was the first to shew, and the observation was repeated by Bonnet {, 

 that the rate of growth or development of the chick was dependent 

 on temperature, being retarded at temperatures below and somewhat 



* Willi Kuhl, Die Variabilitat der abdominalen Korperanhange bei Forficula, 

 Ztsch. Morph. u. Oek. d. Tiere, xii, p. 299, 1924. Cf. Malcolm Burr, Discovery, 

 1939, pp. 340^345. 



t The temperature limitations of life, and to some extent of growth, are sum- 

 marised for a large number of species by Davenport, Exper. Morphology, cc. viii, 

 xviii, and by Hans Przibram, Exp. Zoologie, iv, c. v.^ 



X Reaumur, L'art de faire eclorre et elever en toute saison des oiseaux domestiques, 

 soil par le moyen de la chaleur du fumee, soil par le moyen de celle du feu ordinaire, 

 Paris, 1749. He had also studied, a few years before, the effects of heat and cold 

 on growth-rate and duration of life in caterpillars and chrysalids: Memoires, ii, 

 p. 1, de la dnree de la vie des crisalides (1736). See also his Observations du 

 Thermometre, etc., Mem. Acad., Paris, 1735, pp. 345-376. 



