When sight is the medium of perception movement plays are at the same time visual plays, otherwise consciousness is reached through the sense of touch. We will here give special attention to experimental exercise of the motor apparatus, as actual movement play is treated of in detail in another section. After some general remarks, a few cases will be cited whose most important feature is the pleasure derived from the contemplation of the movement, as is especially the case when it is not self-produced. The powerful attraction which movement has for us is well grounded biologically, for evidently it is of the utmost importance in the struggle for existence that attention should be at once and instinctively aroused by any stir or change in the environment.[141] But perception of movement by means of the eye alone, and consequently the instinct of keeping absolutely motionless, is of great importance to the pursued animal. Thus Edinger says: “I have repeatedly seen a hungry snake pause in the midst of his pursuit of a fleeing mouse, when it crouched down and was quiet. I have seen it recoil from the frog, which it was trying to catch, as soon as the creature kept still.”[142] Even our own involuntary attention to motion has some analogy to instinct, and recalls the violent and sudden reaction with which we respond to an unexpected touch on the bare back.[143] As a matter of psychological fact, there is associated with movement, as with sensations of hearing, a strong emotional effect. It is no wonder, therefore, that all his life long man shows a peculiar interest in movement, and acquires the capacity to detect its intimations very early in life. Indeed, this capacity is one of the first to be developed, and depends, apart from skin stimuli and the so-called after images which reveal objective movement to the eye at rest, principally on the ability to follow the moving object with the glance. Practice is necessary for the mastery of this capacity. The eyes accompany, in addition to the regular objective motion, a constantly renewed backward movement as well, by means of which we again grasp the escaping object, an effort requiring the simultaneous exercise of volition and attention. “This process requiring continuous and constantly renewed attention,” says L. W. Stern, “this lying in wait that the object may not give us the slip (for any laxity would at once be avenged by an increased difficulty in fixing the object), bears witness to a condition and teaches us that the object with which we are carrying on this game of ‘catcher’ is in motion.”[144] This explains why little children so easily lose sight of a moving object which they wish to follow with the eye.