4 - The Cell 



display several unmistakable signs of life. 

 All typical organisms are responsive; that is, 

 all living things react to stimulation — by 

 chemical agents such as food, or by physical 

 agents such as light. Likewise, all living 

 tilings sustain themselves by some kind of 

 nutrition. Each takes in food from which to 

 derive matter lor growth and energy for 

 movement. And above all, each kind of liv- 

 ing thing displays a capacity for reproduc- 

 tion, perpetuating itself from generation to 



nonliving things display activities analogous 

 to nutrition and growth. Water, or any other 

 form of matter, continually alters its be- 

 havior in response to changes in temperature, 

 pressure, light, and other factors of the en- 

 vironment. Water expands or contracts ac- 

 cording to the temperature; it boils at one 

 temperature and freezes at another. These 

 are relatively simple reactions, hardly to be 

 compared to the complex responses of a 

 thinking man, or of a sprouting seed. But 









Fig. 1-2. Responsiveness, nutrition, and reproduction are characteristic of living things. 

 Incidentally, this adult female robin is a fairly rare, white (albino) specimen. (Courtesy of 

 Hugh M. Halliday.) 



generation. In fact, these three activities — 

 responsiveness, nutrition, and reproduction 

 — are uniquely combined in living bodies, 

 and can be taken as the main criteria of the 

 living state (Fig. 1-2). 



Responsiveness. One must conclude, how- 

 ever, that the dividing line between the liv- 

 ing and nonliving is not a very sharp one, 

 because a number of nonliving systems can 

 be found that simulate some aspects of living 

 behavior. All bodies, nonliving as well as liv- 

 ing, are in some degree responsive; and some 



there are nonliving systems that are highly 

 reactive — as for example, a loaded pistol re- 

 sponding to a touch on the trigger, or an 

 automobile responding to pressure on the 

 accelerator. Generally speaking, however, 

 animate responses, in comparison with in- 

 animate ones, are much more complex and 

 variable. 



Nutrition. Nonliving bodies may also dis- 

 play nutrition, although generally the nutri- 

 tion of living bodies is considerabh more 

 complex. Within the living body, food al- 



