GROWTH 



281 



The growth curve of a developing animal has a somev^hat similar 

 form; although there is no definite lag before growth commences, the 

 curve relating mass to time is sigmoid or roughly S-shaped. It was 

 Minot, at about the beginning of the century and following him Brody, 

 who particularly pointed out that it would be more profitable to consider 



the so-called 'specific growth rate' (i.e. growth rate per unit mass -, 



w dt 



which is the same as — -^— ) rather than the simple growth rate {dwidt) 



(Fig. 13.2). As we have just seen, it is only in exceptional circumstances 

 that this can be expected to be constant. Many formulae have been ad- 

 vanced, on a variety of grounds, in an attempt to produce a theoretical 

 scheme which fits the facts better. 



Figure 13.2 

 Various 'growth functions'. The three curves in the left side are concerned 

 with the absolute growth. In a weight is plotted against time to give the 

 curve of growth (a Gompertz equation is assumed) ; in h growth rate is 

 plotted against time, and in c the change of rate (i.e. acceleration) of growth 

 is shown against time. The curves on the right give similar graphs of the 

 functions of the specific growth rate ; that is, in e we plot 'growth rate per 



, 1 dW d\os,W . , r . , dW 



unit mass or— -j- = -j^ — mstead of simple growth rate --.-, and make 



corresponding changes in the other curves d and/. Notice that the specific 



growth rate falls steadily from the beginning of life (curve e), and does so at 



an ever-increasing rate (curve/). (From Medawar 1945.) 



