INTRODUCTION. XXXI 



the motor power of the workman who lifts and carries the blocks ; (2) 

 a blow or other impulse from without applied to the unstable structure- 

 when the atoms will fall together, and as they fall collide with each 

 other and produce heat. Thus, the energy employed by the workman 

 is again transformed into the last-named form of energy. 



In plants, the complex unstable building of the groups of atoms is 

 carried on, the constructor being the sun. In animals, which eat 

 plants, the complex groups of the atoms are tumbled down, with the 

 liberation of kinetic energy. 



Vital Energy and Life. 



The forces which act in organisms, in plants and animals are exactly 

 the same as are recognisable as acting in dead matter. A so-called 

 " vital force," as a special force of a peculiar kind, causing and governing 

 the vital phenomena of living beings, does not exist. The forces of all 

 matter, of organised as well as 'unorganised, exist in connection with 

 their smallest particles or atoms. As, however, the smallest particles of 

 organised matter are, for the most part, arranged in a very complicated 

 way, compared with the much simpler composition of inorganic bodies, 

 so the forces of the organism, connected with the smallest particles, 

 yield more complicated phenomena and combinations, whereby it is 

 excessively difficult to ascribe the vital phenomena in organisms to the 

 simple fundamental laws of physics and chemistry. 



The Exchange of Material, or Metabolism (Stoffwechset) as a Sign of 

 Life. Nevertheless, there appears to be a special exchange of matter 

 and energy peculiar to living beings. This consists in the capacity of 

 organisms to assimilate the matter of their surroundings, and to work 

 it up into their own constitution, so that it forms for a time an integral 

 part of the living being, to be given off again. The whole series of 

 phenomena is called Metabolism or Stoffwechsel, which consists in the 

 introduction, assimilation, integration, and excretion of matter. 



We have already shown, that the metabolism of plants and that of 

 animals are quite different. The processes, as already described, 

 are actually what occur in the typical higher plants and animals. 



But there is a large group of organisms which, throughout their 

 entire organisation, exhibit so low a degree of development, that by 

 some observers they are considered as undifferentiated " ground-forms." 

 They are regarded as neither plants nor animals, and are the most 

 simple forms of animated matter. Hseckel has called these organisms 

 Protistce, as being the original and primitive forms. 



We must assume that, corresponding with their simpler vital condi- 

 tions, their metabolism is also simpler, but on this point we still 

 require further observations. 



