Instinct

Complex though definite and usually involuntary reaction to stimuli of environment: innate tendency of animals, under given conditions of environment, to react in certain definite ways toward the attainment of certain definite ends, these ends being often if not always unrecognized by the animal. Instincts are thus usually spoken of as unintelligent. Their origin is doubtful. They may have arisen by the preservation by habit in succe ive generations of activi ties originally intelligent, or by natural selection, preserving originally unintelligent activities which were useful to the organism.