LIFE AND HER CHILDREN. 7 
pass that the whole planet is used to the best advan- 
tage, and life teaches her children to get all the good 
out of it that they can. 
If the ocean and the rivers be full, then some 
must learn to live on the land, and so we have for 
example sea -snails and land-snails ; and whereas 
the one kind can only breathe by gills in the water, 
the other breathes air by means of air-chambers, 
while between these are some marsh -snails of the 
tropics, which combine both, and can breathe in both 
water and air. We have large whales sailing as 
monarchs of the ocean, and walruses and seals fish- 
ing in its depths for their food, while all other 
animals of the mammalian class live on the land. 
Then, again, while many creatures love the bright 
light, others take advantage of the dark corners 
where room is left for them to live. You can scarcely 
lift a stone by the seaside without finding some 
living thing under it, nor turn up a spadeful of 
earth without disturbing some little creature which 
is content to find its home and its food in the dark 
ground. Nay, many animals for whom there is no 
chance of life on the earth, in the water, or in the 
air, find a refuge in the bodies of other animals and 
feed on them. 
But in order that all these creatures may live, 
each in its different way, they must have their own 
particular tools to work with, and weapons with 
which to defend themselves. Now all the tools and 
weapons of an animal grow upon its body. It 
works and fights with its teeth, its claws, its tail, its 
sting, or its feelers ; or it constructs cunning traps 
by means of material which it gives out from its own 
