SOMATOGENESIS 261 



or less necessary to its life, and the modification of these 

 factors brings about a corresponding variation in the 

 normal progress of somatogenesis. 



These external factors, such as temperature, mois- 

 ture, light, chemical solutions, pressure, etc., may ac- 

 celerate, retard or even inhibit the normal course of 

 events, but invariably such external environmental fac- 

 tors contribute largely to the end result of somatogene- 

 sis. 



It is quite likely that many kinds of monsters and 

 defective organisms are the result not of defective 

 heredity but of alterations in the normal constellation 

 of physical factors which constitute the environment of 

 the developing organism. 



8. THE INTERNAL ENVIRONMENT 



Not only is somatogenesis hedged about by external 

 modifying factors but there is also an internal environ- 

 ment that controls to a large degree the behavior of 

 hereditary factors and determines how they shall come 

 to expression in the somatoplasm. 



The obvious way in which growth is dependent upon 

 the intake and use of food, and the abnormal outcome 

 following an unnatural chemical situation within the 

 body, such as the presence of poisons, are illustrations 

 of what is meant by internal environment. Perhaps the 

 best illustration of this is furnished by the endocrine 

 glands in mammals. Twenty years ago very little was 

 known with certainty about the part that these ductless 

 glands play in the organism, but they have become so 



