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Crustacea are limited by their habit of moulting. A crab as big as a
cow would have to spend most of its life in retirement growing new
armour-plate. Land vertebrates are limited by their skeleton, which
for mechanical reasons must increase in bulk more rapidly than the
animal's total bulk, until it becomes unmanageable. And water
animals are presumably limited by their food-getting capacities.

At last we come to the position of man, as a sizable object, within
the universe. Eddington begins his fascinating Stars and Atoms by
pointing out that man is almost precisely halfway in size between an
atom and a star.
The sun belongs to a system containing some 3000 million stars.
The stars are globes comparable in size with the sun, that is to say,
of the order of a million miles in diameter. The space for their
accommodation is on the most lavish scale. Imagine thirty cricket
balls roaming the whole interior of the earth; the stars roaming the
heavens are just as little crowded and run as little risk of collision
as the cricket balls. We marvel at the grandeur of the stellar
system. But this probably is not the limit. Evidence is growing
that the spiral nebulae are *eisland universes'* outside our own
stellar system. It may well be that our survey covers only one unit
of a vaster organization.
A drop of water contains several thousand million million
million atoms. Each atom is about one hundred-millionth of an
inch in diameter. Here we marvel at the minute delicacy of the
workmanship. But this is not the limit. Within the atom are the
much smaller electrons pursuing orbits, like planets round the sun,
in a space which relatively to their size is no less roomy than the
solar system.
Nearly midway in scale between the atom and the star there is
another structure no less marvellous—the human body. Man is
slightly nearer to the atom than to the star. About io27 atoms
build his body; about io28 human bodies constitute enough
material to build a star.
We can pursue this train of thought a little further. The size-range
of living beings, the amount by which the big tree is bigger than the
filter-passer, is lo24; in other words, the biggest single organism is a
quadrillion times larger than the smallest. Among different phyla
only one has a range over half as great, and this is the unexpected
group of the Protozoa. Molluscs and coelenterates have a range
of io11, and vertebrates, arthropods, and worms one of io10—ten