THE RECAPITULATION THEORY 293 



in place of being situated; as in other fish> one on 

 each side of the head, are both on the coloured 

 side. The advantage to the fish is clear, for the 

 natural position of rest of a flat fish is lying on the 

 sea bottom; with the white surface downwards and 

 the coloured one upwards. In such a position an 

 eye situated on the white surface could be of no 

 use to the fish, and might even become a source of 

 danger, owing to its liability to injury from stones 

 or other hard bodies on the sea bottom. No one 

 would maintain that flat fish were specially created 

 as such. The totality of their organisation shows 

 clearly enough that they are true fish, akin to 

 others in which the eyes are symmetrically placed 

 one on each side of the head, in the position they 

 normally hold among vertebrates. We must there- 

 fore suppose that flat fish are descended from other 

 fish in which the eyes are normally situated. 



The Recapitulation Theory supplies a ready test. 

 On employing it i.e., on studying the development 

 of the flat fish we obtain a conclusive answer. 

 The young sole on leaving the egg is shaped just 

 as any ordinary fish, and has the two eyes placed 

 symmetrically on the two sides of the head. It is 

 only after the young fish has reached some size, 

 and has begun to approach the adult in shape, and 

 to adopt its habit of resting on one side on the sea 

 bottom, that the eye of the side on which it rests 

 becomes shifted forwards, then rotated on to the 

 top of the head, and finally twisted completely over 

 to the opposite side. 



The brain of a bird differs from that of other 



