Irritability and contractility are primary properties of protoplasm. In their earliest forms they express the simple play of elementary chemical and physical forces. In undifferentiated protoplasm, they exist as general diffused properties of reacting to incident forces indifferently in all directions. The progressive differentiation of form and function expressive of organic progress implies the development of specific irritability or susceptibility to specific excitants, along with the capacity to react in definite ways to these excitants. It implies further the development of a nervous mechanism by which this special irritability is manifested, and through which the reactionary impulses are transmitted to be developed into organic activities. The special senses are special developments of this irritability, and the varied motor activities of animals are derivatives of this primary contractility manifested by protoplasm.
All organic reactions involve a reflex element or factor in that they are in a sense responses to stimuli