76 PLANTS AND ANIMALS 
favourable for plankton life may contain several 
millions of individuals in every litre (about 1? pints); 
while as a fair estimate for the seas which surround 
our own islands “at least one” organism for every 
drop has been suggested.* 
In the great abysses of the ocean, where vegetable 
life is absent, the strange creatures which live there 
in utter darkness prey upon others, and they again on 
others which belong to lesser depths, the ultimate 
source of life being again the minute surface organ- 
isms which, possessing chlorophyll, can make organic 
out of inorganic substances by the energy obtained 
from sunlight. Thus only is life made possible in 
the green hells of the sea 
Where fallen skies and evil hues and eyeless creatures be. 
On the land, the dependence of animals on plants is 
in large measure direct, as the supply of vegetable 
food is abundant and widespread. The largest land 
animals are all vegetable feeders; so are the majority 
of our own native mammals, and in a great measure 
our birds; while most of the creatures upon which the 
flesh-eating animals prey are themselves vegetable 
feeders. The distribution of land animals over the 
globe is thus dependent in large measure on the dis- 
tribution of plants. On account of the profusion and 
variety of plant life, and the fact that most vegetable 
feeders can thrive on various sorts of plants, few 
animals are restricted in their range by the presence 
or absence of any particular species or genus, but 
complete dependence of this sort is by no means un- 
* See A. H. Cuurcu: ‘‘ The Plankton-phase and the Plankton- 
rate,’’ Journal of Botany, June, 1919, supplement. 
