Bawven, Psychological Theory of Evolution. 273 
entirely dependent upon it for food. A large number of animals 
are carnivorous, it is true, but these prey upon herbivorous 
animals which, for their part, live entirely upon plants. One 
of the problems which the animal has to face, then, is the prob- 
lem of nutrition in all its various forms. This problem of food 
will be a different one for the flesh-eating than for the plant-eat- 
ing animal. This is represented in the difference in the struc- 
ture of the two types. The ox has a gut which is thirty times 
its own length, while the tiger or lion has a gut which is only 
eight times its own length. This is a rough index of the amount 
of energy required for the digestion of the two kinds of food. 
But an even greater problem was that which the herbivor- 
ous animal had to face when it passed from an aquatic to a ter- 
restrial life. The primitive plant-animal or animal-plant was 
unquestionably an aquatic form. Life first appeared in unicel- 
lular organisms which floated on the surface of the primordeal 
sea where surface stretches of water were the first habitable 
spots. Such an organism existed in a comparatively homogen- 
eous environment where its food supply was constantly at hand, 
requiring at most the development of only very simply organs to 
adapt it in its environment. On the surface of the ocean there 
was an even distribution of light and heat, as well as of food sup- 
ply, so that there was no necessity for the development of 
special sense organs for perceiving the distant object or of com- 
plicated organs of locomotion for reaching it. But in contrast 
with this, imagine what was required when the animal passed 
from the water to the land. Its food no longer lies about it 
evenly distributed in a homogeneous liquid medium. Yet such 
a medium is absolutely essential to the continued existence of a 
living cell. How shall the animal be enabled to live a terrestrial 
existence and yet retain a liquid medium for its growing cells? 
How the problem was solved is obvious in the structure of any 
multicellular animal. The liquid environment was transferred 
within the organism. The cells of the human body, for ex- 
ample, are as really water forms as is the diatom or the amoeba. 
They are embathed in lymph. The leucocyte and red blood 
corpuscle even retain the primitive characteristics in some re- 
