REFLEXES, CONSCIOUSNESS AND WILL
EXTERNAL agents of various types, in addition to their own internal impulses, bring about in living organisms a variety of acts which, in general, promote the survival of the individual or species. Such reactions are in their most diverse applications favourable to the development of life.
This is the case with tropisms in particular among very many possible examples. Plants shoot forth their foliage into the air -- negative geotropism -- and thrust their roots into the ground -- positive geotropism -- under the influence of gravity. The climbing plants, through the action of thigmotaxis, wind around tree-trunks or other solid objects which serve them as support as they stretch forwards. Another frequent occurrence is phototropism or bending towards the light by plants, and we could mention countless other instances.
Such automatic positioning comes about through the agency of certain factors particularly sensitive to definite impulses, located in the cytoplasmic extensions of the epidermal cells of climbing species, or in the hairs or tactile protuberances of Drosera (Sundew), Dionaea (Venus's Fly-trap), Mimosa (Sensitive Plant), and others among the coleoptiles in general. Material factors of metabolic or hormonic nature may also come into play.
In even the most primitive independent forms in the animal world, and even among plants as well, motor reactions to various stimuli are observable, acts of taxis aroused by light, heat, osmotic pressure, electric energy of various kinds, or chemical substances. Movements are steered in one direction or another, approaching or fleeing from the stimulus, according to the nature of the latter and the condition of the organism itself. There is a huge variety of tactism, and study thereof has permitted the discovery that movements, even in the rudimentary forms of life, evolve in accordance with a standard of usefulness, as if answering a particular purpose, namely sheer survival.
Phenomena develop in the best possible manner, which caused Binet ( 1904) and Jennings ( 1906) to say that, in the protozoa and infusoria,