i8o THE RIVER-SIDE NATURALIST. 



but in the lower forms these relative dimensions become 

 smaller and the structure less complicated, until in fishes, 

 which are supposed by some to be the least intelligent 

 of this great division of the animal kingdom, they are 

 found in such a rudimentary state that they are frequently 

 inferior in size even to the olfactory or optic ganglia of 

 the higher forms. Yet, with all this, some fish show a 

 great amount of intellectual discrimination some more 

 than others. A trout, for instance, which always feeds 

 near the surface, soon learns the difference between the 

 shadow or the substance of man and that of other verte- 

 brates. It will, when feeding, take no notice of any 

 animal by the side of the stream or the shadow of a 

 passing cloud ; but let a man, or his shadow or even the 

 shadow of a rod pass near it, the fish will at once be 

 scared either rush madly away, or, in fishing parlance, 

 will be put down. It is the same, only in a less degree, 

 with all ; but as some remain for the most part in deep 

 water, they are less alarmed than those which have to find 

 their food near the surface, and which appear to be able 

 to discriminate between those which can harm them and 

 those which cannot ; and, like all ferce naturce, become 

 more wary the more they come in contact with man, the 

 destroyer of them all. 



As the vision of fish is of great importance to their 

 safety and welfare, the construction of the eye is worthy 

 of consideration. 



The second pair of ganglia in the brain of a fish give 

 origin to the optic nerve, each nerve being composed of 

 a broad band, folded up like a fan, and enclosed in a dense 

 membrane. The eye itself differs in many points from that 

 of terrestrial animals. "Its organisation," says Professor 

 Rymer Jones, " being, of course, adapted to bring the rays 

 of light to a focus upon the retina in the denser element in 

 which the fish resides, the power of the crystalline lens is 

 therefore increased to the utmost extent, and the anterior- 

 posterior diameter of the eyeball necessarily contracted in 

 the same ratio, in order that the retina may be placed 

 exactly in the extremely short focus of the powerful lens. 



