nsumed and explodes without
being blown to bits. It is characteristic of the organism that it
remains a going concern for a longer or shorter period--its length of
life. Living creatures that expended their energy ineffectively or
self-destructively would be eliminated in the struggle for existence.
When a simple one-celled organism explores a corner of the field seen
under a microscope, behaving to all appearance very like a dog scouring
a field seen through a telescope, it seems permissible to think of
something corresponding to mental endeavour associated with its
activity. This impression is strengthened when an amoeba pursues
another amoeba, overtakes it, engulfs it, loses it, pursues it again,
recaptures it, and so on. What is quite certain is that the behaviour of
the animalcule is not like that of a potassium pill fizzing about in a
basin of water, nor like the lurching movements of a gun that has got
loose and "taken charge" on board ship. Another feature is that the
locomotor activity of an animalcule often shows a distinct
individuality: it may swim, for instance, in a loose spiral.

But there is another side to vital activity besides acting upon the
surrounding world; the living creature is acted on by influences from
without. The organism acts on its environment; that is the one side of
the shield: the environment acts upon the organism; that is the other
side. If we are to see life whole we must recognise these two sides of
what we call living, and it is missing an important part of the history
of animal life if we fail to see that evolution implies becoming more
advantageously sensitive to the environment, making more of its
influences, shutting out profitless stimuli, and opening more gateways
to knowledge. The bird's world is a larger and finer world than an
earthworm's; the world means more to the bird than to the worm.

The Trial and Error Method

Simple creatures act with a certain degree of spontaneity on their
environment, and they likewise react effectively to surrounding stimuli.
Animals come to have definite "answers back," sometimes several,
sometimes only one, as in the case of the Slipper Animalcule, which
reverses its cilia when it comes within the sphere of some disturbing
influence, retreats, and, turning upon itself tentatively, sets off
again in the same general direction as before, but at an angle to the
previous line. If it misses the disturbing influence, well and good; if
it str