Of these nerves one set proceed forth from the central part of the nervous system to the different muscles, which they can cause to contract by a peculiar action they exert upon them, thus producing motion. Another set of nerves proceed inwards, from the skin to the central part of the nervous system, and by their peculiar action give rise to various sensations, according as different influences or stimulations are brought to bear upon the skin at, or in the vicinity of, their peripheral extremities. Under ordinary circumstances, different stimulations of the surface of the body convey an influence inwards, which produces sensation, and give rise to an outwardly proceeding influence to the muscles, resulting in definite and appropriate motions. [1] It is often associated unfairly with the illustrious name of the late Mr. Darwin. His special views lend themselves indeed to Haeckelianism, and have been pressed into its service; yet they are by no means to be identified therewith. As Professor Huxley has pointed out with his usual lucidity and force, Darwin’s theory can be made to accord with the most thoroughgoing teleology. [2] See Todd’s _Cyclopædia of Anatomy and Physiology_, vol. iii. p. 3. There are cases in which responsive actions take place under very abnormal conditions—as after a rupture of part of a man’s spinal cord, or the removal of the whole brain in lower animals, such as the frog. A man so injured may have utterly lost the power of feeling any stimulation—pricking, cutting, or burning—of his legs and feet, the injury preventing the conveyance upwards to the brain of the influence necessary to ordinary sensation, and stopping short at the spinal cord below the point of injury. Nevertheless, such a man may execute movements in response to stimuli just as if he did feel, and often in an exaggerated manner. He will withdraw his foot if tickled with a feather just as if he felt the tickling, which he is utterly incapable of feeling. Similarly a decapitated frog will make with his hind legs the most appropriate movements to remove any irritating object applied to the hinder part of its body. Such action is termed “reflex action,” on the supposition that the influence conveyed inwards by nerves going from the skin to the spinal cord is reflected back from that cord to the muscles by the other set of nerves without any intervention of sensation. This action of the frog may be carried to a very singular extreme. At the breeding season the male frog tightly grasps the female behind her arms, and to enable him the more securely to maintain his hold, a warty prominence is then developed on the inner side of each of his hands. Now if such a male frog be taken, and not only decapitated, but the whole hinder part of the body removed also, so that nothing remains but the fragment of the trunk from which the two arms with their nerves proceed, and if under these circumstances