244 PROBLEMS OF RELATIVE GROWTH 



the growth of the part and the growth of the rest of the body. 

 The role of hormones and of mutation in differential growth 

 has been discussed, and the extent of our ignorance on this 

 subject emphasized. 



Finally, the bearings of the study of differential growth on 

 other branches of biology have been discussed, and it has been 

 shown that light is thereby shed upon such diverse problems 

 as orthogenesis, recapitulation, vestigial organs, the existence 

 of non-adaptive characters, physiological genetics, comparative 

 physiology, and systematics. 



I may conclude as I began, by a quotation from D'Arcy 

 Thompson, to whose classical work all students of relative 

 growth owe so much [Growth and Form, p. 719) : 



" The study of form may be descriptive merely, or it may become 

 analytical. We begin by describing the shape of an object in the 

 simple words of common speech : we end by defining it in the precise 

 language of mathematics ; and the one method tends to follow the 

 other in strict scientific order and historical continuity". 



