GENERAL CHARACTERS OF LIVING ORGANISMS 41 



sponses involving metabolic synthesis are called forth 

 under the same conditions as the other more familiar 

 types of response, such as muscular contraction in 

 animals, which depends more directly upon processes of 

 metabolic breakdown. 



The importance of the relations existing between 

 normal growth and the normal physiological activity of 

 the organism has been hitherto insufficiently recognized. 

 Probably the main reason for this is that in the egg and 

 early embr^^o the development of any organ up to a 

 certain stage necessarily precedes its functional activity; 

 often, in fact, development is complete before there is any 

 possibility of function (generative organs, many muscular 

 mechanisms). In many other cases, however, normal 

 physiological activity is a prerequisite for normal 

 growth and development. Inactivity means lowered 

 or subnormal metabolism, and this involves subnormal 

 growth; frequently, when physiological activity is 

 subnormal, metabolic construction lags behind destruc- 

 tion, and regression or atrophy ("disuse-atrophy") 

 results. The need of activity for normal growth is most 

 evident in the adult stages of higher organisms, and is 

 especially well shown in intermittently active tissues 

 like voluntary muscle, where increased activity leads to 

 increased growth, as shown in the effects of exercise, 

 while disuse is followed by regression more or less 

 complete. Other tissues show similar conditions; the 

 removal of one kidney is followed by increase in the 

 size of the other, in correlation with the enforced increase 

 of activity; and valvular insufficiency in the heart leads 

 to muscular enlargement. Such cases of compensatory 

 hypertrophy are apparently an example of the above- 



