388 NOTES. 



then, for it is the work of the brain, not of the auditory nerve. Sound is 

 the sensation produced by the wave-movement of the air. If thus defined 

 in terms of sensation, light is nothing; without eyes the world would be 

 wrapped in darkness. Some Protozoa have a pigment spot as an eye. 



94 In Invertebrates and aquatic Vertebrates, the crystalline lens is globu- 

 lar; or, in other words, it is round in short-sighted animals, and flattish in 

 the long-sighted. The lens of the Invertebrate is not exactly the same as 

 the lens of the Vertebrate eye, though it performs the same function ; it ie 

 really a part of the cornea. 



95 The Ant has fifty in each eye, the House-fly four thousand, the Dragon- 

 fly twenty-eight thousand. 



' 6 The pigment, therefore, while apparently in front of the retina, is really 

 behind it, as in Vertebrates. The layer beneath the cornea, serving as an 

 "iris," is wanting in nocturnal Insects, since they need every ray of light. 

 The optic nerve alone is insensible to the strongest light. 



97 It should be noticed that this corresponds with another peculiar fact 

 already mentioned, that either hemisphere of the brain controls the muscles 

 on the opposite side of the body. In Invertebrates, the motor apparatus is 

 governed on its own side. 



98 Sharks have eyelids, while Snakes have none. The third eyelid (called 

 nictitating membrane') is rudimentary in many Mammals. 



99 An infant would doubtless learn to walk if brought up by a wild beast, 

 since it was made to walk. Just as an Infusorium moves its cilia, not be- 

 cause it has any object, but because it can move them. New-born puppies, 

 deprived of brains, have suckled ; and decapitated Centipedes run rapidly. 

 Such physical instincts exist without mind, and may be termed "blind im- 

 pulses." 



100 We say "apparently," because it may be a fixed habit, first learned by 

 experience, transmitted from generation to generation. A duckling may go 

 to the water, and a hound may follow game in some sense, as Sir John Her 

 schel takes to astronomy, inheriting a taste from his father. Breeders take 

 advantage of this power of inheritance. 



101 We may divide the apparently voluntary actions of animals into three 

 classes. First, organic, in which consciousness plays no part, and which are 

 due wholly to the animal machine. Second, instinctive, in which conscious- 

 ness may be present, but which are not controlled by intelligence. Third, 

 associative, in which the animals act under conscious combination of distinct, 

 single ideas, or past impressions. To these we may add rational acts, in 

 which the mental process takes place under the laws of thought. 



102 "Thus, while the human organism may be likened to a keyed instru- 

 ment, from which any music it is capable of producing can be called forth at 

 the will of the performer, we may compare a Bee, or any other Insect, to a 

 barrel-organ, which plays with the greatest exactness a certain number of 

 tunes that are set upon it, but can do nothing else." CARPENTER'S Mental 

 Physiology, p. 61. This constancy may be largely due to the uniformity of 

 conditions under which Insects live. 



108 We may say, as a rule, that the proportion of instinct and intelligence 

 in an animal corresponds to the relative development of the spinal cord and 



