152 Dynamic Theory. 



toward the left, with teeth opposite the eyes only. The flat fishes are 

 whitish on the under side and dark-colored on the upper side. The 

 upper side also possesses the "Chromatic function," or the power to 

 change color, which the under side does not. Sometimes, however, an 

 individual is colored alike on both sides and is called a Double. The 

 dark side is doubled oftener than the light side. 



Occasionally an individual is " reversed," that is, lies and swims and 

 has his eyes, &c. , on the side opposite the one his species usually 

 affect. 



The embryo flat fish is straight and symmetrical, with eyes in the 

 normal place, but the young one soon begins to assume the sequences of 

 distorted forms that the habits of his ancestors have bequeathed to his 

 race and ends by being as crooked as the rest. This embryonic develop- 

 ment is indicative of the race development and proves the flat fishes to 

 be derived from a symmetrical ancestry and modified subsequently into 

 crookedness. 



Thus the flat fish is a good example of the effects of an unequal 

 stimulus ; that is, the stimulus on one side differing from that on the 

 other, as, with the bottom of the ocean on one side and the upper world 

 on the other, it obviously does. 



The occasional "doubles" and "reversions" illustrate the fact 

 noticed elsewhere that the latest variations from a former type are 

 themselves liable to vary. 



Fig. 81. 



Brain of Halibut, a Flat Fish. 

 A Top view. 

 B Bottom. 



c Optic nerve from left eye. 

 d-Optic nerve from right eye. 

 e Right optic lobe of brain in connection 

 with left optic nerve and left eye. 

 /Left optic lobe of brain connecting 



with right eye. 



g gr The hypoaria-the right one the larg- 

 est. 



a Cerebellum below which is the taper- 

 ing Medulla oblong ata and upper end 

 of spinal cord. (Owen.) 



The difference of strain or attention, exertion and activity forced up- 

 on opposite sides of the body, and especially the eyes, has wrought a 

 great difference in the size of the eyes and of the optic nerves, and even 

 of the parts of the brain relating to the eyes. This is shown in Fig. 81. 

 In this case the left eye is the most active, and the optic nerve leading 

 from it to the brain is far the larger ; and in like manner the correspond- 



