ikes it again, the tactics are repeated until a satisfactory way
out is discovered or the stimulation proves fatal.

It may be said that the Slipper Animalcule has but one answer to every
question, but there are many Protozoa which have several enregistered
reactions. When there are alternative reactions which are tried one
after another, the animal is pursuing what is called the trial-and-error
method, and a higher note is struck.

There is an endeavour after satisfaction, and a trial of answers. When
the creature profits by experience to the extent of giving the right
answer first, there is the beginning of learning.

[Illustration: DIAGRAM OF A SIMPLE REFLEX ARC IN A BACKBONELESS ANIMAL
LIKE AN EARTHWORM

1. A sensory nerve-cell (S.C.) on the surface receives a stimulus.

2. The stimulus travels along the sensatory nerve-fibre (S.F.)

3. The sensory nerve-fibre branches in the nerve-cord.

4. Its branches come into close contact (SY^{1}) with those of an
associative or communicating nerve-cell (A.C.).

5. Other branches of the associative cell come into close contact
(SY^{2}) with the branches or dendrites of a motor nerve-cell (M.C.).

6. An impulse or command travels along the motor nerve-fibre or axis
cylinder of the motor nerve-cell.

7. The motor nerve-fibre ends on a muscle-fibre (M.F.) near the surface.
This moves and the reflex action is complete.]

[Illustration: _Photo: British Museum_ (_Natural History_).

THE YUCCA MOTH

The Yucca Moth, emerging from her cocoon, flies at night to a Yucca
flower and collects pollen from the stamens, holding a little ball of it
in her mouth-parts. She then visits another flower and lays an egg in
the seed-box. After this she applies the pollen to the tip of the
pistil, thus securing the fertilisation of the flower and the growth of
the ovules in the pod. Yucca flowers in Britain do not produce seeds
because there are no Yucca Moths.]

[Illustration: INCLINED PLANE OF ANIMAL BEHAVIOUR

Diagram illustrating animal behaviour. The main line represents the
general life of the creature. On the upper side are activities implying
initiative; on the lower side actions which are almost automatic.

_Upper Side._--I. Energetic actions. II. Simple tentatives. III.
Trial-and-error methods. IV. Non-intelligent experiments. V.
Experiential "learning." VI. Associative "learning." VII. Intelligent
behaviour. VIII. Rational conduct (man).

_Lower Side._--1. Reactions to en