1 36 LIFE AND HER CHILDREN. 
Although we have as yet studied only the lowest, 
and by no means the most numerous of Life's 
children, yet we begin to see that our earth is full, 
very full, of life, and that the creatures in it are 
jostling each other, and driving into dark and 
dismal corners those which cannot get a living in 
the open sunshine. Millions serve as food for 
others, and millions die a speedy death from want 
of space and food ; but we cannot expect that any 
will give up their lives while they can find a means 
of struggling on. What way is there, beyond those 
which we have found already ? 
There is still the novel device of a creature find- 
ing shelter by making another living being carry it, 
and of obtaining food by making another living 
being nourish it. And so we find that among the 
low forms of many classes of animals there are 
always some which prey upon their neighbours, just 
as in our great cities there are always some of the 
most degraded and miserable our street Arabs and 
our thieves who live on refuse and plunder. 
And this is true to such a large extent in the 
animal world, that there is probably scarcely a single 
creature that does not carry many other creatures 
upon or within its body. 
Some of these merely come to it for shelter, as, 
for example, the tiny pea-crab, which is constantly 
found living in the shell of the horse-mussel, catching 
its own food, and being probably rather helpful than 
otherwise to the mussel, by leaving him the scraps 
of his meal. Others, such as ticks and water-mites, 
fix themselves on the bodies, the one of sheep and 
dogs, the other of water-beetles, and sucking the 
