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 terres, is the riddle of the physical basis of lire and of the incentive to evolution in the forms of lire. Not that these mvsteries 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 lire processes in a more efficient manner, and the different groups of living things are the result of the different methods that lire has tried and found advan- tageous for accolnplishing its ends. Living organisms are machines that have become more and more complex in structure, but always for doing the same things. If animals may be compared with machines in their physical mechani?m, 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 lire, the replacement of individuals, opens another subject involving ways and means, and it, likewise, can be understood best in its simpler manifestations.
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INSECTS
