I/o Viewpoints in the Study of Growth [Jan, 



its expression in conclusions such as that the younger the child the 

 more readily is rectiperation accompHshed. It appears to be a fact 

 of experience that animals retarded in growth by underfeeding, as 

 well as yoting children recovering from prolonged illness, begin to 

 grow with more than normal vigor on a return to normal diet and 

 health conditions and at the end of the growing period may be as 

 heavy and have as heavy a brain as their normal companions. Nev- 

 ertheless this will not yet justify us in concluding that the restored 

 individuals are in all respects normal. Qualitative changes not ap- 

 preciated by the cursory examinations may have become perma- 

 nently engrafted. 



In all of the foregoing discussion of the phenomena of growth, 

 its varied aspects, the criteria, the modifying factors and related phe- 

 nomena, little has been said of the Initiation of this fundamental 

 manifestation of living organisms. What is the underlying cause 

 of growth? Like other biological processes this one can as yet be 

 defined satisfactorily only in terms of its manifestations. The 

 cause of regeneration is loss of body substance. It may frankly be 

 admitted at the outset that we know almost nothing in regard to 

 what takes place in protoplasm during growth and very little regard- 

 ing the causes which incite or inhibit it. The only justification for 

 veiling this ignorance in a vague terminology lies in the help which 

 f ormulated hypothesis often brings to the Solution of obscure Prob- 

 lems. With this prefatory Statement growth may be defined as 

 the resultant of an inherent growth impulse — an internal factor — 

 and a suitable environment, the latter including the food supply — 

 an external factor. In these are concerned certain typical biological 

 forces, the metabolism of matter and energy, as well as certain 

 physio-chemical reactions. The conditions determining growth are 

 mainly resident in the cells. 



If we are unable today to define the internal factor suitably, it 

 is equally impossible to cite all the external factors as we do for 

 plants. Air, light, warmth, food, etc., have their functions. How- 

 ever essential food may be for growth — and no one can gainsay its 

 preeminent importance — it can in no sense be regarded as the su- 

 preme cause of growth. Nutrition can only give the growth im- 

 pulse free play. This factor can, however, be subjected to experi- 

 mental analysis. The role of the individual nutrients, — organic and 



