THE PERCEPTION OF SPACE. (II.) 189

the point were moving as well. If the reader will touch his forehead with his forefinger kept motionless, and then rotate the head so that the skin of the forehead passes beneath the finger's tip, he will have an irresistible sensation of the latter being itself in motion in the opposite direction to the head. So in abducting the fingers from each other ; some may move and the rest be still, but the still ones will feel as if they were actively separating from the rest. Vierordt's inferences may be rash, but his experiments certainly show to one who will repeat them how much more like an inde- composable sensation our perception of motion is, than like a constructive act of the mind. But the most valuable contribution to the subject is the paper of G. H. Schneider, 1 who takes up the matter zoologi- cally, and shows by examples from every branch of the animal kingdom that movement is the quality by which animals most easily attract each other's attention. The instinct of ' shamming death ' is no shamming of death at all, but rather a paralysis through fear, which saves the insect, crustacean or other creature from being noticed at all by his enemy. It is paralleled in the human race by the breath-holding stillness of the boy playing ' I spy,' to whom the seeker is near ; and its obverse side is shown in our in- voluntary waving of arms, jumping up and down, and so forth, when we wish to attract someone's attention at a dis- tance. Creatures ' stalking ' their prey and creatures hid- ing from their pursuers alike show how immobility diminishes conspicuity. In the woods, if we are quiet, the squirrels and birds will actually touch us. Flies will light on stuffed birds and stationary frogs. 2 On the other hand, the tre- mendous shock of feeling the thing we are sitting on begin to move, the exaggerated start it gives us to have an insect unexpectedly pass over our skin or a cat noiselessly come and snuffle about our hand, the excessive reflex effects of tickling, &c., show how exciting the sensation of motion is per se. A kitten cannot help pursuing a moving ball. Impres- sions too faint to be cognised at all are immediately felt if they move. A fly sitting is unnoticed, we feel it the moment it crawls. A shadow may be too faint to be per- ceived. As soon as it moves, however, we see it. Schneider found that a shadow, with distinct outline, and directly fixated, could still be perceived when moving, although its objective 1 Vierteljahrssch. fiir wiss. Philos., ii. 377. 2 Exiier tries to show that the structure of the faceted eye of articulates adapts it for perceiving motions almost exclusively.