Man In The Modern World

MAN IN7 THE MODERN WORLD
VI
We come back again to the advantages and disadvantages of size.
At the outset, it is not until living units are quit of the frenzy of
Brownian movement that they themselves become capable of accur-
ately regulated locomotion. The first desirable step in size is to
become so much bigger than ordinary molecules that you can forget
about them.
But even then you are still microscopic, still wholly at the mercy of
anything but the most imperceptible currents. Only by joining to-
gether tens or hundreds of thousands of cells can you begin to make
headway against such brute forces. About the same level of size is
necessary for any high degree of organization to be achieved. Size
also brings speed and power, and this is of advantage in exploring
more of the environment. But the effective range (apart from in-
voluntary floating with the wind or the current) of any creature below
about half a million cells and a hundredth of a gram is extremely
limited. Ants with fixed nests make expeditions of several hundred
yards, and mosquitoes migrate for a mile or so. When we get to
whole grams, however, winged life at least has the world before it.
Many migratory birds that regularly travel thousands of miles weigh
less than ten grams. Swimming life soon follows suit; think of the
migrations of tiny eels across the Atlantic, or of baby salmon down
great rivers. Most land life lags a little; though driver ants are
always on the move, and mice shift their quarters readily enough,
controlled migration hardly begins in land animals till weight is
reckoned by the pound.
If a certain size is needed for any degree of emancipation from
passive slavery to the forces of environment, it is equally needed to
achieve active control over them. Before anything worthy of the
name of brain can be constructed, the animal must consist of tens of
thousands of cells. The insects with best-developed instincts run from
a milligram to a gram. But while a very efficient set of instincts can
be built up with the aid of a few hundred or thousand brain-cells,
rapid and varied power of learning demands a far greater number.
For instincts are based on fixed and predetermined arrangements of
nerve-paths, while efficient learning demands the possibility of almost
innumerable arrangements. The facts are that no vertebrates of less
than several grams weight (such as small birds) show any power of
rapid learning, and none below several ounces weight (such as rats)
are what we usually call intelligent, while even the smallest human
dwarf lias a body-weight to be reckoned in tens of pounds. We are
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