DIV. ii , PHYSIOLOGY 217 



of the plant as it is in that of a machine. In the organism we 

 are concerned not with the mechanical interaction of parts but with a 

 succession of chemical reactions. While it is true that the phenomena 

 of life cannot as yet be thoroughly explained, this does not negative 

 the conviction that they only differ from the processes in inorganic 

 bodies by their much greater complexity ; in principle a physico- 

 chemical explanation of vital phenomena can be attained. 



(b) Capacity of Regulation. The study of machines not only 

 assists in the comprehension of a liberating stimulus but further 

 renders clear the second widely-spread property of organisms, i.e. their 

 regulative power. As in a machine the speed may be automatically 

 maintained at a particular level, so in numerous processes in a plant 

 there is an element which controls the result both as regards quality 

 and quantity. Though self-regulated processes are not wanting in 

 the inorganic world, they do not occur abundantly as they do in 

 the organism. ON THIS ACCOUNT THE POWER OF REGULATION MAY 



BE REGARDED, TOGETHER WITH THE IRRITABILITY, AS A SPECIALLY 

 IMPORTANT CHARACTERISTIC OF LIVING BEINGS. 



4. So long as the organism is actively living, an unbroken chain 

 of changes can be recognised in it which are exhibited in the three 

 following ways : 



(i.) An organism, which appears to us as an individual, does not 

 consist of the same unchanged material, even when no further growth 

 in size is taking place. While its external form remains constant, 

 progressive changes go on internally. New substances are taken 

 up from without, are transformed within the plant, and are again given 

 off from it. The organism has a METABOLISM. Inorganic nature 

 offers us no process analogous to this. 



(ii.) As a rule, however, metabolism does not proceed so that the 

 absorption and giving-off of material are equal, but more is absorbed 

 than is given off. The mass of the organism is increased, it GROWS. 

 Growth is also known in the cases of chemical precipitates or deposits, 

 and of crystals. In these cases it tends to proceed in such a way 

 that no essential change of shape takes place (crystals), or that the 

 changes in shape are accidental and irregular (precipitates). The 

 organism, on the other hand, by changes of its form assumes quite 

 definite shapes, which follow in regular order. It passes through a 

 DEVELOPMENT which leads sooner or later to the production of new 

 organisms or daughter individuals ; REPRODUCTION takes place. 

 Growth, development, and reproduction are processes highly charac- 

 teristic of living beings. 



Some precipitates have a certain external similarity to plants under certain 

 conditions. If some sulphate of copper to which sugar has been added is intro- 

 duced into a solution of ferrocyanide of potassium and common salt containing 

 gelatine, a precipitate of ferrocyanide of copper is formed. This to all appearance 

 grows, and in its form recalls that of plants. This "artificial plant" lacks, however, 



