GENERAL CHARACTERISTICS 19 



The actual nature of the light and the manner in which it is 

 produced are but ill understood, but the variations and fitfulness of 

 its appearances can be to a certain extent conjectured from our 

 knowledge of some of the animals that produce it. 



In our own seas the luminosity is undoubtedly caused princi- 

 pally by the presence of myriads of minute floating or free- 

 swimming organisms that inhabit the surface waters. Of these 

 each one has its own season, in which it appears in vast numbers. 

 Some appear to live entirely at or near the surface, but others 

 apparently remain near the surface only during the night, or only 

 while certain conditions favourable to their mode of life prevail. 

 And further, it is possible that these minute creatures, produced 

 as they generally are in vast numbers at about the same time, and 

 being more or less local, are greatly influenced by changes of 

 temperature, changes in the nature of the wind, and the periodic 

 changes in the tides ; and it is probable that we are to look to these 

 circumstances for the explanations of the sudden changes so fre- 

 quently observed. 



In warmer seas the phenomenon of phosphorescence is much 

 more striking than in our own, the brilliancy of the light being 

 much stronger, and also produced by a greater variety of living 

 beings, some of which are of great size, and embrace species 

 belonging to the vertebrates and the higher invertebrate animals. 



Those interested in the investigation of this subject should make 

 it a rule to collect the forms of life that inhabit the water at a 

 time when the sea is unusually luminous. A sample of the water 

 may be taken away for the purpose of examination, and this 

 should be viewed in a good light, both with and without a magni- 

 fying lens. It is probable, too, that a very productive haul may be 

 obtained by drawing a fine muslin net very slowly through the 

 water. After some time the net should be emptied and gently 

 washed in a small quantity of sea water to remove the smaller 

 forms of life contained, and the water then examined at leisure. 



Of course it must not be assumed that all the species so obtained 

 are concerned in any way with the phosphorescence of the sea, but 

 any one form turning up in abundance when collected under 

 the conditions named will probably have some connection with 

 the phenomenon. 



One may well ask ' What is the use of this light-emitting power 

 to the animals who possess it ? ' but this question is not easily 

 answered. The light produced by the glow-worm and other 



