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 environment. 2. Enregistered reactions. 3. Simple reflex actions. 4. Compound reflex actions. 5. Tropisms. 6. Enregistered rhythms. 7. Simple instincts. 8. Chain instincts. 9. Instinctive activities influenced by intelligence. 10. Subconscious cerebration at a high level (man).] [Illustration: _Photo: J. J. Ward, F.E.S._ VENUS' FLY-TRAP