THE PRIMARY CONDITIONS OF ANIMAL LIFE 11] 
as determined by animals so existing. But it is impossible 
to conceive of animals which could live without oxygen or 
without organic food. The necessities of oxygen and organic 
food (and water) are the primary or essential conditions 
for the existence of any animals. 
Of course, we might include such conditions, among 
the primary conditions, as the light and heat of the sun, 
the action of gravitation, and other physical conditions 
without which existence or life of any kind would be im- 
possible on this earth. But we here consider by “ primary 
conditions of animal life” rather those necessities of living 
animals as opposed to the necessities of living plants. 
Neither animals nor plants could exist without the sun, 
whence they derive directly or indirectly all their energy. 
66. Difference between animals and plants.—lIt is easy to 
distinguish between the animal and plant when a butterfly 
is fluttering about a blossoming cherry tree or a cow feed- 
ing in a field of clover. It is not so easy, if it is, indeed, 
possible, to say which is plant and which is animal when 
the simplest plants are compared with the simplest ani- 
mals. It is almost impossible to so define animals as to 
distinguish all of them from all plants, or so to define 
plants as to distinguish all of them from all animals. 
While most animals have the power of locomotion, some, 
like the sponges and polyps and barnacles and numerous 
parasites, are fixed. While most plants are fixed, some of 
the low aquatic forms have the power of spontaneous loco- 
motion, and all plants have some power of motion, as espe- 
cially exemplified in the revolution of the apex of the 
growing stem and root, and the spiral twisting of tendrils, 
and in the sudden closing of the leaves of the sensitive 
plant when touched. Among the green or chlorophyll- 
bearing plants the food consists chiefly of inorganic sub- 
stances, especially of carbon which is taken from the car- 
bonic-acid gas in the atmosphere, and of water. But some 
green-leaved plants feed also in part on organic food. 
