UNIT TWO — REVIEW • UNDER WHAT CONDITIONS CAN WE LIVE? 



We all feel that "life" is the central and the important thing in the world. 

 We often speak of "life" as if it were.a peculiar something or being which 

 happens to dwell in certain natural objects, but which might as well exist 

 elsewhere, or not at all. Yet what we know of "life" is what we can observe 

 and understand about the activities of living plants and animals. These 

 plants and animals, in turn, continue to be alive — to "have life" — only under 

 rather special circumstances. 



There are many kinds of substances in the world — some ninety elements 

 and numberless compounds. Certain of these are present in all living things. 

 A few are present occasionally, in a few species; and some are never found 

 in living things, or may even be injurious. But in every case life goes on 

 only on condition that these few elements are available — or rather certain 

 of their compounds. 



Living forms are found in all zones of the earth, in the waters and on 

 the mountains, and in the deserts too. But everywhere water is an essential 

 material condition of life. At the same time, water may be a source of 

 injury. It is not merely that some of us might drown if completely sub- 

 merged, but for various plants and animals an excess of water means a 

 diluting of the intake, or a bloating of the tissues. 



These materials contribute both to the bodies of living things and to the 

 processes that characterize plants and animals. These constant chemical 

 changes are in a sense both the processes of living and the conditions of 

 living. These chemical processes continue under a wide range of physical 

 circumstances. Each species, however, can live only within relatively re- 

 stricted ranges. Thus living things exist close to the freezing point of water 

 at one extreme and near the boiling point at the other. It is only the very 

 simplest types of organisms that endure such extremes of temperature — 

 different species at each extreme. But many of the back-boned animals are 

 adapted to a wide temperature range by special protective coverings and by 

 complex mechanisms that keep the inside of the body at a nearly uniform 

 temperature. 



Light influences protoplasm in various ways — even injuriously, when of 

 extreme intensity. And yet it is upon sunlight that the whole world of 

 plants and animals ultimately depends for its nourishment. For this form 

 of energy makes possible the construction of carbohydrates out of water and 

 carbon dioxide. And plants and animals utilize these compounds, first as 

 sources of energy for their own activities, and second as bases for the pro- 

 teins out of which new protoplasm is constantly being made. 



The million or more different species, and the countless individuals in 

 each species, all depend upon essentially the same basic conditions. All 



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