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THE AMERICAN NATURALIST [Vol. XLI1 



veloped to the greatest refinement. By its use organic 

 movements can be recorded graphically, and can then be 

 easily analyzed into their space and time components and 

 be studied at leisure. The working of the organs of the 

 mammalian body, considered as physical machines, is 

 now fairly well understood, although specific problems 

 within this field are still being actively investigated. 

 Very exact computations have been made of the amount 

 of energy given off by the body in the form of heat 

 and of muscular work, and it has been found to cor- 

 respond very closely with the income of energy derived 

 from the fond and whatever bodily material may be con- 

 sumed during the experiment. The principle of the 

 conservation of energy applies as well to the living as to 

 the non-living machine. 



Chemical physiology, or, as it is now often called, bio- 

 chemistry, developed gradually during the last century 

 but did not become prominent until the last decade. It 

 occupies now a foremost place among the branches of 

 biological science. Much biochemical work is morpho- 

 logical; the determination of the chemical constituents 

 and structure of substance once living, from which infer- 

 ences may be drawn as to the chemical nature of living 

 substance. Unfortunately, living substance can not be 

 chemically analyzed directly, since all known methods at 

 once kill it, and there is left only the non-living proteins, 

 carbohydrates, fats and other organic and inorganic com- 

 pounds, the individual bricks, or, better, cleavage 

 products of the complex unity. In determining these and 

 their relationships great progress has been made, but we 

 of the present are far removed from that state of smug 

 satisfaction of some of the earlier investigators, to whom 

 a living body represented only so many molecules of car- 

 bon, oxygen, nitrogen, hydrogen, sulphur and phos- 

 phorus. The problems of the chemical physiologist, as 

 distinguished from the chemical morphologist, are in gen- 

 eral the problems of metabolism,— which the Germans 

 have aptly styled " Staff 'ir echsel' " '—the chemical changes 



