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Mahomet and the mountain meet halfway. With the biological
invention of a blood-system, this need for branching disappears. The
enormous area of surface which is needed is now furnished by the
linings of innumerable tiny vessels, and the organs themselves can
revert to a compact form. Finally, insects and spiders have developed
a breathing system which supplies air direct to the tissues, providing a
large surface for gas exchange in the tiny end branches of the air-
tubes, which penetrate even into the individual cells.
In swimming and flying, too, surface comes into play. No large
animal could move with sufficient rapidity by means of the micro-
scope " hairs" we call cilia, since the size of a single cilium can never be
more than microscopic, and their number depends on the extent of
surface. The largest animals provided with cilia are new-hatched
tadpoles, and all they can achieve is an exceedingly slow gliding.
When muscles are employed in swimming, their force must be
applied to the water through the intermediary of some surface—the
body may be wriggled, or its motions communicated to an enlarge-
ment at the tail, or limbs developed as oars or paddles. When the
animal is small, these swimming surfaces are relatively so big that
little or no special adaptations are needed; but once it grows bulky,
the swimming surface must be enlarged. The body itself is expanded
sideways, as in leeches; or up and down, as in sea-snakes; a regular
tail-fin is developed, as in most fish; or the limbs are expanded into
flat plates, as in turtle or swimming-crab.
The necessary increase of surface in swimming limb or tail can at
iirst be achieved by stiffening and multiplying hairs and spines; but
as soon as the animal exceeds a few millimetres in length this ceases
to be enough, and the organ itself must be expanded. The change is
beautifully seen within the individual development of many crus-
The same applies to wings. All flying animals more than a fraction
of a gram in weight require a broad and continuous expanse to fly
with, whether this be a sheet of skin, as in bats, a marvellous corn-
pound structure such as the whig of a birda or the thin hinged flap
of an insect's wing. But if they are much smaller, a double row of
hairs on either side of a central rod will serve perfectly well. This is
seen in some minute insects, such as the littie thrips, which include
several plant pests, and some tiny wasps which parasitize other
insects' eggs. The lovely plume-moths are a little larger, and are
intermediate in wing construction; their flight surface is made of
hairs, but it is only rendered sufficient by a multiplication of the
number of hair-fringed rods.