PROBLEM OF MIGRATION. 443 
migration. Plants can use the carbon dioxide in the air only 
in the presence of sunlight. Perhaps it would be better to state 
now that all energy comes from the sun; in the presence of 
carbon dioxide plants can and do fix this energy, and turn it 
into living protoplasm, and this energy is passed on to the 
animal, and by it converted into heat and motion. Those who 
have not followed the progress of biology are apt to look upon 
the above asa species of unscientific mysticism, but indeed it is 
the groundwork of biology, and has long since ceased to be 
matter for speculation. 
A remarkable point about the carbon dioxide of the atmo- 
sphere is that it remains constant in its proportion to the rest 
of the air. Roughly speaking, there are four parts by volume 
to ten thousand parts of air, but this actually varies from °037 to 
"062 per cent.—a variation of about ‘04 per cent. For our pre- 
sent purposes we can state that the amount of carbon present in 
the atmosphere remains invariable, although animals are cease- 
lessly adding to it, and plants are removing it. This suggests 
that the ratio between plant life and animal life remains rigidly 
constant from month to month and year to year. What this 
ratio may be I cannot say, but from a long study of balanced 
aquaria I should assume that by weight the vegetation of the 
earth is at the very least several hundred times greater than the 
animal life. 
The plant needs no power of locomotion, for, rooted in the 
soil, water, air, and sunlight will flow to it and provide it with 
food. A little thought shows that, as the food of the animal is 
many times greater in size, locomotion becomes the first essential 
to the growing animal. At the least it must move about the 
mass of its pabulum, and this power of movement has been 
developed to a high degree. The typical animal may be de- 
scribed as a plant-eating mouth and stomach provided with loco- 
motive powers, and its movements are due to the potential energy 
from the sun gathered and stored in the tissues of the plant. 
Three things are essential for plant life—soil, water, and sun- 
light. Under certain conditions the first may be dispensed with, 
but the other two never. When all three conditions are present, 
plants flourish. Latitude has little to do with the question of 
vegetation, and the countries within the Arctic Circle are often 
2m 2 
