26 SENESCENCE AND REJUVENESCENCE 



The transmission of stimuli in living tissues is also very com- 

 monly regarded as dependent in some way upon the colloid con- 

 dition, although here again there are differences of opinion as to 

 the exact nature of the process. 



Our knowledge of the colloids and particularly of the organic 

 colloids is far from complete; undoubtedly the future will clear up 



i many points which are now obscure, but even now it is clear that 

 the colloid substratum in which the chemical reactions of metab- 



1 olism occur is an essential factor in making the phenomena of life 

 what they are. Bechhold ('12), referring to the possibility of life 

 on other planets, asserts that whatever the substances may be which 

 make up*such organisms they must be colloids. In fact, the more 

 we know concerning colloids the less possible it becomes to con- 

 ceive of anything similar to what we regard as life apart from them. 

 Whatever else it may be, it seems certain that the organism is a 

 colloid system. From this point of view our definition of a living 

 organism must be somewhat as follows: A living organism is a 

 ^ ; f specific complex of dynamic changes occurring in a specific colloid 



| substratum which is itself a product of such changes and which 



\ influences their course and character and is altered by them. 



THE RELATION BETWEEN STRUCTURE AND FUNCTION 



The definition of the organism given above leads us to very 

 definite conclusions concerning the relation between structure and 

 function. 



The dynamic processes which occur in organisms do not and 

 cannot constitute life in the absence of the colloid substratum, nor 

 is the colloid substratum alive without the dynamic processes. 

 But since the colloids characteristic of the organism are among the 

 products of the dynamic processes, it is also evident that the pro- 

 cesses cannot go on in their entirety without producing the colloid 

 substratum. In other words, neither structure nor function is 

 conceivable except in relation to each other. 



The beginning of life is to be sought neither in a particular 

 complex of chemical reactions nor in a special morphological struc- 

 ture. Both the reactions and the colloid substratum are necessary 

 for life. But since the substratum is formed in the course of the 



