ELEMENTARY VITAL PHENOMENA 253 



which they are based and their relationship in comparison with 

 all other modes of motion. 



The contraction of living substance follows the same principle 

 everywhere, whether the living substance creeps about as an 

 Amaba upon decaying leaves in a pool of water, whether as a 

 white blood-corpuscle it forces its way through lymph spaces in 

 the tissues of the animal body, whether as a protoplasmic network 

 it circulates in the cellulose-capsule of a plant-cell, whether as a 

 muscle-fibre it performs the contractions of the untiring human 

 heart, or, finally, as a cilium on the oviduct of woman it trans- 

 ports the unfertilised egg-cell to the uterus to undergo fertilisa- 

 tion, everywhere there is the same phenomenon of alternating 

 contraction and expansion of the living substance by means of the 

 reciprocal rearrangement of its particles. 



2. The Production of Light 



In the movements of living substance, especially in the 

 phenomena of contraction, the transformation into kinetic energy 

 of the potential energy introduced into the body as food, comes 

 out very clearly. This is much less evident in the production of 

 other forms of kinetic energy, such as light, heat, and electricity, 

 for the demonstration of which very complicated methods and 

 sensitive instruments are often required. 



Next to the mechanical energy of movement, the production 

 of light is most evident to the senses, and has always had a 

 mysterious fascination for the observer. It has a curious charm, 

 when at night the water of a quiet sea breaks into a bright,, 

 yellow glow at every stroke of an oar, or when in southern 

 climates in the spring, the mild night air is filled by innumerable 

 sparks, which silently flash up and circle about, and then 

 disappear. 



The emission of light by living substance is wide-spread. It is 

 an especially significant fact that, of the wonderful pelagic animals 

 whose delicate transparent bodies occupy the upper strata of the 

 sea and float about as plankton, almost all possess luminous 

 power. Associated with this fact is the presumption that the 

 luminous capacity of living substance is possibly much wider- 

 spread than is realised, that we do not see the light because the 

 organisms are not transparent, or because the production is too 

 feeble to allow the light to be seen through thick body-layers; 

 indeed, it is not impossible that in our own bodies certain cells 

 may be photogenic. In most cases, as in luminous insects, the 

 power of emitting light is a peculiarity specially perfected by 

 selection and possesses its own significance for the life of the 

 animals in question. In pelagic marine animals also such a 

 significance is certainly present ; as a rule, these animals emit light 



