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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.

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-