AND ITS INHABITANTS 85 



It must suffice then to emphasize that it is the presence of 

 proteins and the power of forming them which is the diag- 

 nostic chemical characteristic of matter which is alive. It will 

 be recognized, of course, that proteins are not "aHve," and even 

 if the biochemist should succeed in artificially synthesizing a 

 protein, which has not thus far been accomplished, by group- 

 ing together amino acids, which already have been made syn- 

 thetically, this would not be in any sense the artificial produc- 

 tion of life. But since proteins are the one molecular system of 

 which we are aware which is diagnostic of protoplasm, we have 

 the right to assume, for the present at least, that they represent 

 in a way the quintessence of that arrangement of matter which 

 exhibits life, while clearly recognizing that they are but an 

 integral part of the more complex, physicochemical structure 

 of protoplasm. Protein is, as it were, "the chemical nucleus 

 or pivot around which revolve a multitude of reactions char- 

 acteristic of biological phenomena." 



The vehicle of life manifestations is thus clearly a physico- 

 chemical complex and one of its most conspicuous and funda- 

 mental phenomena is what has been described as "energy 

 traffic" or the function of trading in energy. This consists in 

 appropriating energy from the environment, storing it in a 

 state of higher potential and later expending it in the kinetic 

 form. The lability of many of the molecular aggregates of 

 protoplasm, that is, their tendency to change in composition 

 toward greater or less complexity, coinciding with deoxidative 

 and oxidative processes, is an important characteristic and one 

 which endows the protoplasmic system as a whole with its 

 perennial plasticity — "its peculiar proneness to change its com- 

 position under the stimulus of slight changes in the energy- 

 equilibrium between itself and its surroundings."^ 



Definitions of life. This continual flux, which is now em- 



8 Allen, F. J., "What is Life?" Proc. Birmingham Nat. Hist, and Philos. Soc., 

 vol. 11, pp. 44-67, 1899. 



