this betrayal seems at first sight like a failure of adaptation.
Certainly many a rabbit must be spotted and shot, or killed by birds of
prey, solely on account of that tell-tale white patch as he makes for
his shelter. Nevertheless, when we come to look closer, we can see, as
Mr. Wallace acutely suggests, that the tell-tale patch has its function
also. On the first alarm the parent rabbits take to their heels at once,
and run at any untoward sight or sound toward the safety of the burrow.
The white patch and the hoisted tail act as a danger-signal to the
little bunnies, and direct them which way to escape the threatened
misfortune. The young ones take the hint at once and follow their
leader. Thus what may be sometimes a disadvantage to the individual
animal becomes in the long run of incalculable benefit to the entire
community.

It is interesting to note, too, how much alike in build and gait are
these three thoroughbred desert roamers, the giraffe, the ostrich, and
the camel or dromedary. In their long legs, their stalking march, their
tall necks, and their ungainly appearance they all betoken their common
adaptation to the needs and demands of a special environment. Since food
is scarce and shelter rare, they have to run about much over large
spaces in search of a livelihood or to escape their enemies. Then the
burning nature of the sand as well as the need for speed compels them to
have long legs which in turn necessitate equally long necks, if they are
to reach the ground or the trees overhead for food and drink. Their feet
have to be soft and padded to enable them to run over the sand with
ease; and hard horny patches must protect their knees and all other
portions of the body liable to touch the sweltering surface when they
lie down to rest themselves. Finally, they can all endure thirst for
long periods together; and the camel, the most inveterate
desert-haunter of the trio, is even provided with a special stomach to
take in water for several days at a stretch, besides having a peculiarly
tough skin in which perspiration is reduced to a minimum. He carries his
own water-supply internally, and wastes as little of it by the way as
possible.

What the camel is among animals that is the cactus among plants--the
most confirmed and specialised of desert-haunting organisms. It has been
wholly developed in, by, and for the desert. I don't mean merely to say
that cactuses resemble camels because they are clumsy, unga