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 
