III. _Animal theory of consciousness._--All animals, and they alone, have consciousness. This theory would draw a sharp distinction between the psychic life of the animal and of the plant. Such a distinction was urged by many of the older writers, and was clearly formulated by Linné in his celebrated _Systema Naturae_; the two great kingdoms of the organic world are, in his opinion, divided by the fact that animals have sensation and consciousness, and the plants are devoid of them. Later on Schopenhauer laid stress on the same distinction: "Consciousness is only known to us as a feature of animal nature. Even though it extend upwards through the whole animal kingdom, even to man and his reason, the unconsciousness of the plant, from which it started, remains as the basic feature. In the lowest animals we have but the dawn of it." The inaccuracy of this view was obvious by about the middle of the present century, when a deeper study was made of the psychic activity of the lower animal forms, especially the coelenterates (sponges and cnidaria): they are undoubtedly animals, yet there is no more trace of a definite consciousness in them than in most of the plants. The distinction between the two kingdoms was still further obliterated when more careful research was made into their unicellular forms. There is no psychological difference between the plasmophagous protozoa and the plasmodomous protophyta, even in respect of their consciousness. IV. _Biological theory of consciousness._--It is found in all organisms, animal or vegetal, but not in lifeless bodies (such as crystals). This opinion is usually associated with the idea that all organisms (as distinguished from inorganic substances) have souls: the three ideas--life, soul, and consciousness--are then taken to be coextensive. Another modification of this view holds that, though these fundamental phenomena of organic life are inseparably connected, yet consciousness is only a part of the activity of the soul, and of the vital activity. Fechner, in particular, has endeavored to prove that the plant has a "soul," in the same sense as an animal is said to have one; and many credit the vegetal soul with a consciousness similar to that of the animal soul. In truth, the remarkable stimulated movements of the leaves of the sensitive plants (the mimosa, drosera, and dionæa), the automatic movements of other plants (the clover and wood-sorrel, and especially the hedysarum), the movements of the "sleeping plants" (particularly the _papilionacea_), etc., are strikingly similar to the movements of the lower animal forms: whoever ascribes consciousness to the latter cannot refuse it to such vegetal forms.