106 THE AMERICAN NATURALIST [Vol. XLVII 



a problem must end unsuccessfully in vague and unprofit- 

 able guesses. But the physico-chemical basis of life it at 

 length firmly established. On the whole, the composition 

 of living matter, its physical structure, the changes of 

 matter and energy which constitute the metabolic process, 

 together with the totality of such changes, which make up 

 the fundamental economic process of that largest com- 

 munity which consists of all living beings, are all clearly 

 defined. 



The Characteristics of Life 

 Under these circumstances it is certainly no rash enter- 

 prise to seek a definition of some of the essential char- 

 acteristics of life.. Although it is probably far beyond 

 our present power to make a complete study of the prob- 

 lem, I feel sure that a brief analysis will justify certain 

 very definite conclusions. Life as we know it is a physico- 

 chemical mechanism, and it is probably inconceivable 

 that it should be otherwise. As such it possesses, and, 

 we may well conclude, must ever possess, a high degree 

 of complexity — physically, chemically and physiologi- 

 cally, that is to say, structurally and functionally. We 

 can not imagine life which is no more complex than a 

 sphere, or salt, or the fall of rain, and, as we know it, it is 

 in fact a very great deal more complex than such simple 

 things. Next, living things, still more the community of 

 living things, are durable. But complexity and dur- 

 ability of mechanism are only possible if internal and 

 external conditions are stable. Hence automatic regula- 

 tions of the environment and the possibility of regulation 

 of conditions within the organism are essential to life. 

 It is not possible to specify a large number of conditions 

 which must be regulated, but certain it is from our present 

 experience that at least rough regulation of temperature, 

 pressure, and chemical constitution of environment and 

 organism are really essential to life, and that there is 

 great advantage in many other regulations and in finer 

 regulations. Finally a living being must be active, hence 

 its metabolism must be fed with matter and energy, and 



