dusk of the long summer day, alighting only for
brief moments at the nest to deliver insects to the young. All the
active life of bats certainly deserves to be called aerial.

The air was the last haunt of life to be conquered, and it is
interesting to inquire what the conquest implied. (1) It meant
transcending the radical difficulty of terrestrial life which confines
the creatures of the dry land to moving on one plane, the surface of the
earth. But the power of flight brought its possessors back to the
universal freedom of movement which water animals enjoy. When we watch a
sparrow rise into the air just as the cat has completed her stealthy
stalking, we see that flight implies an enormous increase of safety. (2)
The power of flight also opened up new possibilities of following the
prey, of exploring new territories, of prospecting for water. (3) Of
great importance too was the practicability of placing the eggs and the
young, perhaps in a nest, in some place inaccessible to most enemies.
When one thinks of it, the rooks' nests swaying on the tree-tops express
the climax of a brilliant experiment. (4) The crowning advantage was the
possibility of migrating, of conquering time (by circumventing the arid
summer and the severe winter) and of conquering space (by passing
quickly from one country to another and sometimes almost girdling the
globe). There are not many acquisitions that have meant more to their
possessors than the power of flight. It was a key opening the doors of a
new freedom.

The problem of flight, as has been said in a previous chapter, has been
solved four times, and the solution has been different in each case. The
four solutions are those offered by insects, extinct Pterodactyls,
birds, and bats. Moreover, as has been pointed out, there have been
numerous attempts at flight which remain glorious failures, notably the
flying fishes, which take a great leap and hold their pectoral fins
taut; the Flying Tree-Toad, whose webbed fingers and toes form a
parachute; the Flying Lizard (_Draco volans_), which has its skin pushed
out on five or six greatly elongated mobile ribs; and various "flying"
mammals, e.g. Flying Phalangers and Flying Squirrels, which take great
swooping leaps from tree to tree.

The wings of an insect are hollow flattened sacs which grow out from the
upper parts of the sides of the second and third rings of the region
called the thorax. They are worked by powerful muscles, and are
s