But little observation is required to convince us of the fact that, in the higher creatures, a very simple stimulus may give rise to a very complex response. A light pin-prick will cause a vigorous leap in a healthy frog--a leap that involves a most intricate, accurate, and complex co-ordination of muscular activities. And anatomical investigation shows us that in such creatures there is always, in the course of the channel of communication or transmission, a group of closely connected cells, which play the part of co-ordinants. In the vertebrate animals these co-ordinants are collected in the brain and spinal cord. In the insects, crustaceans, and worms they are arranged in a knotted chain running close to the under surface of the body. To this central nervous system, as it is called, nerves (afferent nerves) run inwards from the recipient organs. From it nerves (efferent nerves) run outwards to the organs of response. And in it the transmitted stimuli, brought in by the afferent nerves, are modified, through intervention of the co-ordinants, into stimuli carried out by the efferent nerves. A simple stimulus may create a great commotion among the co-ordinants of the central nervous system, and give rise to many and complex stimuli going out to the muscles and other organs of response. How this is effected is one of the many wonders of the animal mechanism. We believe that the connection and co-ordinations have gradually been established during a long process of development and evolution, reaching back far into the past. How, we can at present scarcely guess.