HEREDITY FROM PHYSICO-CHEMICAL POINT OF VIEW. 79 



degrees; and any theory of growth or heredity must assign some 

 definite basis for this characteristic conservatism. We have 

 suggested above that this basis is the tendency of similarly con- 

 stituted compounds to segregate in the formation of aggregates, 

 and in this way to form structures which determine the direction 

 of metabolism. But such an hypothesis explains only the resem- 

 blance of an organism to its parent; it does not indicate how the 

 proliferative process itself is carried out. It appears, in fact, 

 that two separate groups of problems are involved in the theory 

 of heredity the one relating to the conditions determining the 

 resemblance to the parent stock, the other relating to the nature 

 of the physiological mechanism by which the living substance 

 (apart from its special nature) increases its quantity or grows at 

 the expense of materials taken from the surroundings. Such 

 growth is a physiological activity requiring the expenditure of 

 energy, and it cannot be considered apart from its relation to 

 the other physiological, i. e., functional, activities of the organism. 

 The nature of this latter relation now calls for special considera- 

 tion. 



The general significance of the normal functional activity of 

 the living system as one of the chief factors in the formation of 

 its characteristic structure, or in structure-forming metabolism 

 generally, has been insufficiently regarded by writers on heredity. 

 In general, any normally active tissue maintains itself or grows, 

 while an inactive tissue remains stationary or undergoes regres- 

 sion, even if supplied with an abundance of oxygen and food- 

 material; this last is well shown in a voluntary muscle whose 

 innervation has been interrupted. It is clear that increased 

 functional activity involves an increase of constructive as well 

 as of destructive metabolism; and conversely the subnormal 

 metabolism accompanying inactivity is associated with sub- 

 normal construction, which may even fall below destruction, 

 with regression or atrophy as a result. The control which func- 

 tion exercises upon the building-up of living structure is seen 

 perhaps most clearly in higher organisms, and especially in 

 tissues like voluntary muscle, whose activity is intermittent 

 and subject to much variation. In this case the effects of exer- 

 cise in promoting growth and of disuse in causing regression are 



