8 THE FIRST PRINCIPLES OF HEREDITY 



new ones, if, as it is scientifically expressed, during the 

 process of Metabolism* the upbuilding process (Anabolism) 

 is outweighed by the breaking-down process (Katabolism), 

 the body wastes away and finally dies. 



Senile decay and natural death are nothing but the ex- 

 pression of the excess of katabolic changes in the body 

 over anabolic changes. 



(b) Regeneration. 



In addition to these phenomena of life and growth, 

 where a process of what may be called physiological re- 

 generation is involved, we find the organism endowed with 

 a still greater power — that of regenerating whole parts of 

 the body under exceptional circumstances. It is this capa- 

 bility to replace lost parts of the body in toto which is 

 generally comprised under the name of Regeneration proper. 



It is a well-known fact of daily life that a wound heals ; 

 a defect in the skin, due to an accident, is rapidly repaired. 

 What is this healing process ? Nothing but a process of 

 cell-regeneration, akin to the one described above. What 

 happens is this : The cells in the neighbourhood of the 

 injury are stimulated to activity ; they propagate, multiply, 

 and speedily fill up the gap with young cells, each kind of 

 cell regenerating its own kind. This, within certain limits, 

 is the process involved, whatever may be the part of the 

 body concerned — within certain limits only, because the 

 repair is not always complete. The higher the organism, 

 the greater the differentiation of its cells, the less capable 

 these become, after completely attained growth in the 

 adult body, to reproduce themselves in mass. The possi- 

 bility of regeneration depends largely on the organization 

 attained by the species concerned. It is for this reason 

 that we find the phenomena of regeneration exhibited to 



* Metabolism is the process of assimilation in the body, by which 

 the food taken into the body is changed and worked up into the 

 organic substance of the cell. 



