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Full text of "Man In The Modern World"

tarantulas not exceeding two or three ounces. The pigmy among
animal groups is that of the rotifers or wheel animalcules, the most
gigantic among which fails to weigh ten milligrams! They comprise,
too, the smallest of all multicellular animals, some of their adult males
weighing considerably less than a thousandth of a milligram, so that
it would take about a thousand of them to equal one of our striated
muscle fibres, and over a million of them to weigh as much as a
Even the biggest rotifers are much smaller than the biggest among
the Protozoa, or single-celled animals. Some of the extinct num-
mulites, flattened disc-shaped Foraminifera, were bigger than a
shilling, and must have weighed well over a gram. They easily beat
many small fish and frogs in size, and were bigger than the largest
ants, which, though the most successful of all invertebrates, never
reach one gram in weight, and are usually much less. The largest ant
colonies known possess a million or so inhabitants. This whole
population would weigh about as much as one large man. Indeed,
the small size of most insects is at first hearing barely credible. Three
average fleas go to a milligram. If you bought an ounce of fleas, you
would have the pleasure of receiving over eighty thousand of them.
Even a solid hive-bee weighs less than a gram—over five hundred bees
to the pound, nearly a hundred thousand to outweigh a single
average man I
The lower limit of size among the various groups is much more
constant than the upper. The smallest insects, Crustacea, most groups
of worms, and coelenterates, all lie between one hundredth and one
thousandth of a milligram. Some very primitive worms run down
one class further, and rotifers two. The smallest molluscs, lamp-
shells, and echinoderms are between ten and a thousand times larger,
while the smallest vertebrate is four classes up—ten thousand times as
big. Even so the difference between the maximum sizes attained by
different main groups is greater by a hundred thousand times than
the difference between their minima.
There is clearly a lower limit set to a multicellular animal by the fact
that it must consist of at least several hundred cells. But it seems to
be impossible or unprofitable to construct a vertebrate out of less than
several hundred million cells. The vertebrates, both at top and at
bottom, are the giants of the animal kingdom.
It is a surprise to find a frog that weighs as much as a fox-terrier.
It is still a greater surprise to know that there exist fully formed adult
insects—a beetle or two, and several parasitoid waspHke creatures—
of smaller bulk than the human ovum and yet with compound eyes, a