in the dusk does not knock against branches or other
obstacles. Some say that it is helped by the echoes of its high-pitched
voice, but there is no doubt as to its exquisite tactility. That it
usually produces only a single young one at a time is a clear adaptation
to flight, and similarly the sharp, mountain-top-like cusps on the back
teeth are adapted in insectivorous bats for crunching insects.

Whether we think of the triumphant flight of birds, reaching a climax in
migration, or of the marvel that a creature of the earth--as a mammal
essentially is--should evolve such a mastery of the air as we see in
bats, or even of the repeated but splendid failures which parachuting
animals illustrate, we gain an impression of the insurgence of living
creatures in their characteristic endeavour after fuller well-being.

We have said enough to show how well adapted many animals are to meet
the particular difficulties of the haunt which they tenant. But
difficulties and limitations are ever arising afresh, and so one fitness
follows on another. It is natural, therefore, to pass to the frequent
occurrence of protective resemblance, camouflage, and mimicry--the
subject of the next article.

BIBLIOGRAPHY

ELMHIRST, R., _Animals of the Shore_.
FLATTELY AND WALTON, _The Biology of the Shore_ (1921).
FURNEAUX, _Life of Ponds and Streams_.
HICKSON, S. J., _Story of Life in the Seas_ and _Fauna of the Deep Sea_.
JOHNSTONE, J., _Life in the Sea_ (Cambridge Manual of Science).
MIALL, L. C., _Aquatic Insects_.
MURRAY, SIR JOHN, _The Ocean_ (Home University Library).
MURRAY, SIR JOHN AND HJORT, DR. J., _The Depths of the Ocean_.
NEWBIGIN, M. I., _Life by the Sea Shore_.
PYCRAFT, W. P., _History of Birds_.
SCHARFF, R. F., _History of the European Fauna_ (Contemp. Sci. Series).
THOMSON, J. ARTHUR, _The Wonder of Life_ (1914) and
_The Haunts of Life_ (1921).

IV

THE STRUGGLE FOR EXISTENCE

ANIMAL AND BIRD MIMICRY AND DISGUISE

Sec. 1

For every animal one discovers when observing carefully, there must be
ten unseen. This is partly because many animals burrow in the ground or
get in underneath things and into dark corners, being what is called
cryptozoic or elusive. But it is partly because many animals put on
disguise or have in some way acquired a garment of invisibility. This is
very common among animals, and it occurs in many forms and degrees. The
reason why it is so common is because the strug