30 



PROBLEMS OF RELATIVE GROWTH 



Now it is to be observed that a constant differential growth- 

 ratio, during some at least of the period of growth, is not 

 merely an empirical rule found over a large range of organs 

 and groups, but is what one would expect on a priori grounds. 

 For it is the biologically simplest method we can conceive of 

 obtaining the enlargement (or diminution) of an organ. 



If an organ is to begin its career small and end it large, 

 the obvious method is to make it grow at a higher rate than 

 the rest of the body ; the difference once initiated (by what- 

 ever physiological means) there is no a priori reason why it 

 should not be maintained at approximately the same relative 

 level throughout, since changes which affect the growth of the 

 body will be expected to have, within narrow limits, a pro- 

 portionate effect on the growth of the organ (and see p. 6). 



We shall later consider certain special cases where growth 

 of organ and body appear not to be equally affected in cer- 

 tain circumstances (pp. 200, 259) ; others which show that 

 the formula for constant differential growth-ratio can only be 

 an approximation when we are dealing with an organ as a 

 whole (p. 81) ; and still further facts which rule out some of 

 the early stages of an organ's development from the operation 

 of the law (p. 139, seq.). However, both empirical facts and 

 theoretical considerations warrant us in regarding a constant 

 differential growth-ratio, operating over a longer or shorter 

 period of time, as the primary law of the relative growth of 

 parts, once they have reached the stage of full histological 



