2. Closely allied to the foregoing reason for a belief in the evolution of mind is that derived from a comparative survey of the nervous system in man and other animals. In man, mind is closely associated with a certain tissue or system of tissues—_nerve tissue or the nervous system_. That mind is correlated with nerve structure, and that mental anatomy may be learned from a study of the anatomy of the nervous system, especially of the brain, is the basic postulate of the science of physiological psychology. Now, nerve cells exist in all animals above the sponge, and a comparatively well-developed nervous system is found even among many of the invertebrates, as the higher worms, crustaceans, insects, and mollusks. The nervous system of invertebrates, though composed of the same kind of tissue, is constructed according to a somewhat different plan of architecture from that of the vertebrates. But in all of the great family of backboned animals the nervous system is built on the same general plan as in man, with a cerebro-spinal trunk extending from the head along the back and motory and sensory nerves ramifying to all parts of the body. There is also a sympathetic nervous system in all animals down as far as the insects. The brain, which is the most important part of the nervous system, and which has been called the ‘organ of consciousness,’ presents throughout the animal kingdom, from its beginning in the worms to man, a graduated series of increasing complication proceeding out of the same fundamental type. This is especially true of the vertebrates. Fishes, amphibians, reptiles, birds, and mammals, all have in their brains the same primary parts, the same five fundamental divisions, as are found in the brain of man. Hence, whatever may be thought about the mental states of invertebrates, we have the right, in the case of the vertebrate orders of life, to infer, from the general similarity of their nervous system to our own, that they have a corresponding similarity to ourselves in mental constitution and experience. 3. The evolution of mind is suggested by the existence in the animal world of all grades of intelligence, from almost mindless forms to forms even exceeding in some respects the mental attainments of men. The jelly-fish and the philosopher are not mental aliens. They are linked to each other by a continuous gradation of intermediate intelligences. The existence of these grades of mental development suggest psychical evolution and kinship, just as the existence of like grades of structural development suggest physical evolution. 4. In the mental life of animals the same factors of evolution exist as those by means of which organic structures have been brought into existence, and it is reasonable to suppose that the operation of these factors have produced in the mental world results analogous to those produced by the operation of the same factors among organic structures.