116 NATURE AND LIFE. 



are essentially true. All organic activity was very clearly 

 at first borrowed from the sun, and if the earth has since 

 stored away and made its own a quantity of energy, that 

 sometimes suffices to produce of itself that which originally 

 proceeded from solar stimulus, it must not be forgotten 

 that those living forces, of startling and complex aspects, 

 sometimes our pitiless enemies, often our docile servants, 

 have descended, and are still descending, upon our planet, 

 from the inexhaustible sun. The study of animal life shows 

 us by striking instances the physiological efficacy of light, 

 and the immaterial chain, it may be called, which links ex- 

 istences with the fiery and abounding heart of the known 

 universe. 



In plants, as we have seen, respiration at night is the 

 reverse of that by day. There are infusoria which behave, 

 under the influence of light, exactly like the green portions 

 of plants. These microscopic animalcula are developed in 

 fine weather in stagnant water, and in breathing produce 

 oxygen at the expense of the carbonic acid contained in the 

 liquid. Morren saw that the oxygenation of the water oc- 

 casioned by these little beings varied very perceptibly in 

 the course of twenty-four hours. It is at the minimum at 

 sunrise, and reaches its maximum toward four in the after- 

 noon. If the sky is overcast, or the animalcula disappear, 

 the phenomenon is suspended. This is only an exception. 

 Animals breathe at night in the same way as in the day- 

 time, only less energetically. Day and night they burn 

 carbon within their tissues, and form carbonic acid, only 

 the activity of the phenomenon is much greater in light 

 than in darkness. 



Light quickens vital movements in animals, especially 

 the act of nutrition, and darkness checks them. This fact, 

 long known and applied in practical agriculture, is express- 

 ly noted by Columella. He recommends the process of 

 fattening fowls by rearing them in small, dark cages. The 



