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Full text of "Man In The Modern World"

THE SIZE OF LIVING THINGS

nice nervous system, three pairs of jaws and three pairs of legs, veined
wings, striped muscles, and the rest! It is rather unexpected that the
smallest adult vertebrate is not a fish, but a frog; and it is most
unexpected to find that the largest elephant would have ample clear-
ance top and bottom inside a large whale's skin, while a full-sized
horse outlined on the same whale would look hardly larger than a
crest embroidered on the breat pocket of a blazer.

Then we come to single cells. By far the largest is—or was—the
yolk of the extinct Aepyornis's egg, which must have weighed some
ten pounds. But eggs are exceptional cells; so are multinucleated
cells like striated muscle fibres and the biggest nummulites. Of cells
with a single nucleus, some protists such as Foraminifera may reach
over a milligram—gigantic units or protoplasm; and the ciliate
Bursaria is nearly as big. But among ordinary tissue cells of Metazoa
the largest are only about one hundredth of a milligram, while average
cells of a mammal range between a thousandth and a ten-millionth
of a milligram. In our own frames, the body of a large nerve-cell is
well over ten thousand times bulkier than a red blood-corpuscle or a
spermatozoon—a difference five or ten times greater than that be-
tween the largest whale and the average man. (In these calculations
the outgrowths of the nerve-cells have been left out of account, as
peculiar products of cell activity. If they are included, then the
spinal sensory and motor nerve-cells, supplying the limbs of the giant
dinosaurs and of giraffes, take the palm for size; but even they can
only reach a few milligrams, in spite of being over ten feet long.)

The smallest free-living true cells are in the same size-class with the
smallest tissue cells; but parasitic Protozoa, which live inside other
cells, may be a hundred times smaller. Bacteria are built on a
different scale. The largest of them are little bigger than the smallest
tissue cell, and the average round bacterium or coccus is a thousand
times smaller. These finally pass below the limits of microscopic
vision, until, with the filter-passers, such as the virus of distemper or
yellow fever, we reach organisms with only about a thousand protein
molecules. Somewhere near these we may expect to find the lower
limit of size proscribed to life; for several hundred molecules are
probably as necessary in the construction of an organic unit as are
several hundred cells for the construction of a multicellular animal.

m
Having made a little voyage of discovery among the bare facts, it
is time to begin a quest for principles.   The great bulk of land verte-
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