WAYS AND MEANS OF LIVING 



takes the form of a change of shape in the protoplasmic 

 mass, or movement, we say the mass exhibits signs of life. 

 The state of being alive, however, is more truly shown if 

 the act can be repeated, for the essential property of living 

 matter is its power of reverting to its former chemical 

 composition, and its ability thus gained of again reacting 

 to another change in the environment. In restoring its 

 lost elements, it must get these elements anew from the 

 environment, for it can not take them back from the sub- 

 stances that have been lost. 



Here, expressed in its lowest terms, is the riddle of the 

 physical basis of life and of the incentive to evolution in 

 the forms of life. Not that these mysteries are any more 

 easily understood for being thus analyzed, but they are 

 more nearly comprehended. Being alive is maintaining the 

 power of repeating an action; it involves sensitivity to 

 stimuli, the constant presence of free oxygen, elimination 

 of waste, and a supply of substances from which carbon, 

 hydrogen, nitrogen, and oxygen, or other necessary ele- 

 ments, are readily available for replacement purposes. 

 Evolution results from the continual effort of living matter 

 to perform its life processes in a more efficient manner, and 

 the different groups of living things are the result of the 

 different methods that life has tried and found advan- 

 tageous for accomplishing its ends. Living organisms are 

 machines that have become more and more complex in 

 structure, but always for doing the same things. 



It animals may be compared with machines in their 

 physical mechanism, they are like them, too, in the fact 

 that they wear out and are at last beyond repair. But 

 here the simile ends, for when your car will no longer run, 

 you must go to the dealer and order a new one. Nature 

 provides continuous service by a much better scheme, for 

 each organism is responsible for its own successor. This 

 phase of life, the replacement of individuals, opens another 

 subject involving ways and means, and it, likewise, can be 

 understood best in its simpler manifestations. 



[ioi] 



