THE SIZE OF LIVING THINGS
far from knowing the precise size needed; but the intelligence of a
rat would be impossible without brain-cells enough to outweigh the
whole body of a bee, while the human level of intellect would be im-
possible without a brain composed of several hundred million cells,
and therefore with a weight to be reckoned in ounces, outweighing
the very great majority of existing whole animals. In any case, a very
considerable size was a prerequisite to the evolution of the human
Size too means a disregarding of obstacles: the rhinoceros crashes
through the bush that halts and tangles man; the horse gallops over
the grass that is a jungle to the ant. Size may help to intimidate or
to escape from enemies, or may enable the carnivore to attack new
and larger prey; and it usually goes with longevity.
Size thus holds out many advantages for life. But size brings dis-
advantages as well as advantages, and so life finally comes up against
a limit of size, where disadvantages and advantages balance.
The limits are different for different kinds of animals, for they
depend upon the construction of the type, and upon the world which
it inhabits. Single-celled animals, as we have seen, soon reach a limit
on account of the surface-volume relation. Organisms that must
swim and have only cilia to swim with come to a limit even earlier.
Whether they be one- or many-celled, the limit is at about a milli-
gram. Those which use cilia, not to swim, but to produce a food
current, are not handicapped until much later; by folding the cur-
rent-producing surface, and arranging neat exits and entrances for
the current, many lamp-shells and bivalve molluscs reach several
ounces; but as the current-producing cilia are confined to a surface,
there comes a limit, which is attained when the soft parts reach a
weight of a few pounds.
With most slow-moving sea animals, it is the food question which
restricts size. It is usually more advantageous to the race to have a
number of medium-sized animals utilizing the food available in a
given area than to put all the biological eggs into the single basket of
one big individual. Without some greater degree of motility than
these possess, sea-urchins or sea-cucumbers as big as sheep would be
inefficient at exploiting the food resources of the neighbourhood. The
only such slow creatures above a few pounds weight of soft parts are
jellyfish, the largest of which manage to obtain sufficient food in the
crowded surface waters of cold seas by spreading prodigious nets of
Insects and spiders have so low a limit of size because of their air-
tube method of breathing, which is inefficient over large distances.