NOTES 479 



There is no such thing as sound till the vibrations strike the tympanum, 

 nor even 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, as Euglena, have a pigment 

 spot as an eye. 



148 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 is 

 really a part of the cornea. 



149 The ant has 50 in each eye, the house fly 400x5, the dragon fly 28,000. 



150 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. 



151 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. 



152 Sharks have eyelids, while snakes have none. The third eyelid (called 

 nictitating membrane} is rudimentary in many mammals. It may be seen 

 at the inner angle of the eye. 



153 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 

 because it has any object, but because it can move them. Newborn 

 puppies, deprived of brains, have suckled; and decapitated centipedes run 

 rapidly. Such physical instincts exist without mind, and may be termed 

 " blind impulses." 



154 \Y e 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 

 Herschel devoted himself to astronomy, inheriting a taste from his father. 

 Breeders take advantage of this power of inheritance. 



155 \y e ma y 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 dis- 

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

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



156 "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 



