444 THE ZOOLOGIST. 
covered in summer with a luxuriant srowth of plants. Some 
districts, as the interior of Greenland, are plantless, and, of 
course, tropical deserts may be equally unvegetated. But the 
great extent of the tundren around the North Pole supports, in 
summer only, a wealth of greenery. 
These Arctic plants die down at the approach of winter, for 
the clear reason that life would be impossible, owing to the 
absence of sunlight. We can perhaps, for the sake of simplicity, 
dismiss the soil and the water as factors of plant growth, and 
confine ourselves for the moment to the third ever-present factor, 
solar energy or sunlight. The amount of the plant life of any 
area thus depends upon the amount of sunlight poured upon 
this area. At the Equator, with a regular day of twelve hours’ 
sunlight, the amount remains constant throughout the year, but 
in temperate and Polar regions the amount varies with the 
seasons. In Britain, with eight hours of sunlight in winter 
and sixteen in summer, the difference in vegetation is very 
marked. Within the Arctic Circle, with utter darkness for one- 
half the year and constant daylight for the other half, the con- 
ditions are still more different. In winter the water is congealed 
as ice, and the land is in darkness and buried beneath a cloak 
of snow. In summer the returning sun releases the water, 
removes the covering of snow, and pours down on the awaken- 
ing plants a flood of energy. The necessary carbon dioxide is 
present, as everywhere, in the atmosphere, and the result is the 
amassing of huge stores of potential energy. 
The plants thus share the carbon of the air with the plants 
of other lands, but if the balance of life is to be kept constant, 
they must indirectly return their share to the atmosphere. 
This, of course, can only take place by the medium of animal 
life. Perhaps this is sufficiently clear, without being further 
elaborated, and I would like to discuss now a very delicate 
corner of the subject. Itis obvious that, as all plants share 
equally in the carbon dioxide of the atmosphere and equally 
add the animal requirement of oxygen to the atmosphere, the 
balance of life can only be maintained by all vegetation being 
accessible to animals. The sun (and I beg leave to use here a 
handy freedom of terms) moves slowly towards the North Pole 
from the South Pole, but the plants cannot follow it. The 
