86 EVOLUTION OF THE EARTH 



phasized as characteristic of protoplasm, was recognized as 

 a characteristic of organic individuals long before it was 

 recognized in their physical basis, and is expressed in nearly 

 all definitions of life. Aristotle described life as "the assem- 

 blage of the operations of nutrition, growth and destruction"; 

 Bichat as "the sum total of the functions which resist death"; 

 De Blainville as "the twofold internal movement of composi- 

 tion and decomposition, at once general and continuous"; 

 Lewes as "a series of definite and successive changes, both of 

 structure and composition, which take place within an indi- 

 vidual without destroying its identity"; while Spencer defines it 

 as "the definite combination of heterogeneous changes, both 

 simultaneous and successive, in correspondence with external 

 coexistences and sequences" and as "the continuous adjustment 

 of internal relations to external relations." 



Obviously these statements are far from satisfactory, but 

 no adequate, concise and yet comprehensive definition of life 

 ever has been formulated, though it has been essayed by 

 the best minds for generations. The fact is that life is too 

 complex to be described concisely, and so unique that it is 

 impossible to resort to the lexicographer's trick of comparing 

 it with something else. Although many physicochemical 

 phenomena contribute to life, not one of them will serve as 

 a criterion and all of them collectively do not offer, as yet, a 

 satisfactory explanation. Whether behind the physical phe- 

 nomena there are intangible metaphysical factors is a question 

 outside the realm of biology. Since the limitations of our 

 senses confine our perception to that part of our environment, 

 perhaps a very small part, denoted by the fundamental con- 

 cepts of science, matter, and energy acting in space and time, 

 we are forced to interpret, in so far as we are able, the life 

 processes in these terms. Thus far, however, the study of 

 protoplasm has given no reason for abandoning the productive 

 working hypothesis that life phenomena are an expression of 



