Protoplasm. 47 



light seems to have little or no particular effect. Some 

 animals live throughout their whole existence in darkness, 

 and very many others in light so dim that they may be said 

 to be practically independent of its influence altogether. 

 The special luminosity known as phosphorescence, which 

 many of these animals are themselves capable of producing, 

 may in some part act as a substitute for sunlight. 



E. Food-supply. Since animals have no . power of 

 building up complex organic compounds out of simple 

 inorganic materials, but are dependent directly or indirectly 

 on the vegetal kingdom for their food, manifestly it is of the 

 first importance for the maintenance of animal life that 

 vegetals should have the means of nourishment in abund- 

 ance and in an accessible form. The food of animals is 

 extremely variable. Some animals are entirely herbivorous, 

 others entirely carnivorous, while others still are omnivorous. 

 It must be at once evident, in order that any group of 

 animals may remain in existence, that a sufficient supply of 

 that particular food on which they subsist should be available. 

 In some cases that supply is extremely localised and small 

 in amount. The larvce of certain flies, for example, are de- 

 pendent for the maintenance of their life on the larvae of a 

 particular species of bee. 



The food materials required by plants are most easily 

 determined by analysing the plant itself (p. 187). An ex- 

 amination of the products of desiccation, of calcination, and 

 of the ash shows that the following elements are required 

 for the maintenance of plant life : 



I. and chiefly, carbon, hydrogen, oxygen, and nitrogen. 



II. and less important, sulphur, potassium, calcium, 

 iron, magnesium, phosphorus, chlorine, sodium, and silicon. 



The source of the carbon has already been sufficiently 

 explained ; nitrogen is obtained by the plant from com- 

 pounds of ammonia and nitric acid in the soil, and not as 

 might have been expected from the enormous supply in the at- 

 mosphere; oxygen is obtained by the decomposition of water, 



