Although the animals which especially display these exceptional or contingent instincts are those which are fitted for the use and comfort of man and may be domesticated, it is doubtless true that many other species are in some degree provided with them, and that they thus have a plasticity in their nature which enables them to exercise, under particular circumstances, unlooked-for attention, foresight, and caution. And besides, it is only in analogy with the laws of the physical world that instinct should admit of a slightly diversified application. It is to be noticed in this connection that many animals are gifted with a wonderful sensibility of the senses,--the action of which is sometimes mistaken not only for the action of instinct, but for that of reason also. The acuteness of the sense of smell in the dog, which enables him to trace the steps of his master for miles through crowded streets by the infinitesimal odor which his footsteps left upon the pavement, is quite beyond our conception. Equally incomprehensible to us are the keenness of sight and wide range of vision of the eagle, which enable him to discover the rabbit nipping the clover amid the thick grass at a distance at which a like object would be to us altogether imperceptible. The chameleon is enabled to seize the little insects upon which it feeds by darting forth its wonderfully constructed tongue with such rapidity and with such delicacy of perception that "wonder-loving sages" have told us that it feeds upon the air.