THE CONQUEST OF THE DRY LAND 229 
instance, is one of the strongest of all animals. 
But it is quite different with most of the ani- 
mals that have found their home in caves. 
Many of them are infirm, many are weak- 
eyed, many are nervous and delicate. While 
we admit that some of the cave animals may 
have degenerated because they have lived so 
long in caves, there is much to be said for the 
view that most of the cave animals took to 
the caves because they were weakly. This is 
borne out by the study of animals that have 
recently become cave-dwellers. 
ARBOREAL LIFE 
Animals owe a great deal to plants. In the 
long run they depend on plants for food; ani- 
mals use the munitions which plants manufac- 
ture. Plants prepared the earth for animals, 
making it friendly; they helped to secure 
moisture and soft hiding-places. They formed 
a subtle sieve against which animal life has 
often beat, with the best of results. But one 
of their crowning benefits was in providing 
animals with trees to climb on. We some- 
times use the phrase, “up a tree,” to suggest 
that a man is in a difficult and dangerous 
