INTRODUCTION 



Statement of the problem. The most promising prospect 

 for an understanding' of the mechanisms of growth and 

 postnatal development in man and the higher animals is the 

 analysis and regulation of the internal secretions in conjunc- 

 tion with the genetic constitution of the individual. Advances 

 in knowledge along these lines may open the way for proper 

 control of physical and mental development, as well as of 

 susceptibility to deficiencies and infectious disease. Such 

 possible regulation and control will, of course, always depend 

 to a considerable extent upon the genetic constitutions of the 

 individuals concerned. Certainly all individuals cannot re- 

 spond similarly to exactly the same experiences. This fact 

 in itself demands analysis and explanation. 



It is urgently important at the present stage of our knowl- 

 edge of this subject to attempt an estimate, based upon con- 

 trolled experiments, of the interrelations of the roles played 

 by genetic constitution and the endocrine secretions in both 

 normal and modified types of physical development and func- 

 tional expression. Only a few scattered contributions to this 

 intricate problem have as yet appeared. Some of these show 

 in a definite way that modified conditions in the glands of 

 internal secretion are clearly hereditary and are correlated 

 with structural and functional peculiarities in the animal 

 body. A study of the inheritance of a bulldog type in cattle 

 by Crew ('23), a preliminary statement by the writer ('23 a, 

 '23 b), followed by more recent reports ('26, '27 a, '27 b, '28, 

 '30, '31, '32 a, '32 b, '32 c, '34, '35, '36 a, '36 b, '36 c, '37, 

 '38 a, '38 b) on the significance of dog breeds in this connec- 

 tion, the studies on the creeper-fowl by Landauer ('31, '32, 

 '33) and Landauer and Dunn ('30), the investigation of an 

 hereditary dwarf mutation in mice associated with defective 

 pituitary glands by Smith and MacDowell ('30, '31) and the 





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