242 THE RATE OF GROWTH [ch. 



Now if temperature or light affect the rate of growth in strict 

 uniformity, aHke in all parts and in all directions, it will only lead 

 to local races or varieties differing in size, as the Siberian goldfinch 

 or bullfinch differs from our own. But if there be ever so Httle of a 

 discriminating tendency such as to enhance the growth of one tissue 

 or one organ more than another*, then it must soon lead to racial, 

 or even "specific," difference of form. 



It is hardly to be doubted that chmate has some such dis- 

 criminating influence. The large leaves of our northern trees are 

 an instance of it; and we have a better instance of it still in Alpine 

 plants, whose general habit is dwarfed though their floral organs 

 suffer little or no reduction f. Sunhght of itself would seem to be 

 a hindrance rather than a stimulant to growth; and the familiar 

 fact of a plant turning towards the sun means increased growth on 

 the shady side, or partial inhibition on the other. 



More curious and still more obscure is the moon's influence on 

 growth, as on the growth and ripening of the eggs of oysters, sea- 

 urchins and crabs. Behef in such lunar influence is as old as Egypt; 

 it is confirmed and justified, in certain cases, nowadays, but the 

 way in which the influence is exerted is quite unknown J. 



Osmotic factors in growth 



The curves of growth which we have been studying have a 

 twofold interest, morphological and physiological. To the morpho- 

 logist, who has learned to recognise form as a "function of growth," 

 the most important facts are these: (1) that rate of growth is an 

 orderly phenomenon, with general features common to various 

 organisms, each having its own characteristic rates, or specific 

 constants; (2) that rate of growth varies with temperature, and so 

 with season and with climate, and also with various other physical 

 factors, external and internal to the organism ; (3) that it varies in 

 different parts of the body, and along various directions or axes: 



* Or as we might say nowadays, have a different "threshold value" in one 

 organ to another. 



t Cf. for instance, NageH's classical account of the effect of change of habitat 

 on alpine and other plants, Sitzungsber. Baier. Akad. Wiss. 1865, pp. 228-284. 



X Cf. Munro Fox, Lunar periodicity in reproduction, Proc. R.S. (B), xcv, 

 pp. 523-550, 1935; also Silvio Ranzi, Pubblic. Slaz. Zool. Napoli, xi, 1931. 



