xliv INTRODUCTION. 



As the formation of solar heat and solar light is explicable by the gravitation of 

 masses, gravity is perhaps the original form of energy of all life. 



We may thus represent the formation of kinetic energy in the animal body from 

 the potential energy of plants. Let us suppose the atoms of the substances formed 

 in organisms, as simple small bodies, balls, or blocks. As long as these lie in a 

 single layer, or in a few layers, upon the surface, there is a stable arrangement, 

 and they continue to remain at rest. If, however, an artificial tower be built of 

 these blocks, so that an unstable erection is produced, and the same tower be after- 

 wards knocked down, then for this purpose we require (1) the motor power of the 

 workman who lifts and carries the blocks ; (2) a blow or other impulse from with- 

 out 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 constructer 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 (" Stoffwechsel ") 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, actually occur in the typical 

 higher plants and animals. 



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

 tion, exhibit so low a degree of development, that by some observers they are con- 

 sidered as undifferentiated " ground-forms." They are regarded as neither plants 

 nor animals, and are the most simple forms of animated matter. Haeckel has 

 called these organisms Protistse, as being the original and primitive forms. 



We must assume that, corresponding with their simpler vital conditions, their 

 metabolism is also simpler, but on this point we still require further observations 

 and experiments. 



