THE ARGUMENT FROM EMBRYOLOGY 85 



flat fish such as the sole, flounder, turbot, and 

 plaice, which are distinguished not merely by the 

 remarkable flattening of the body from side to side, 

 but by the further facts — (1) that the two sides, right 

 and left, of the fish are never coloured alike, one 

 being nearly white, and the other dark-coloured ; and 

 {2) that the two eyes, instead of being situated, as in 

 other animals, one on each side of the head, are both 

 on the same side — i.e., the darkly-coloured one. On 

 watching these flat fish in an aquarium, we note that 

 they habitually lie on the bottom on the paler 

 coloured side, and we are at once led to associate the 

 remarkable condition of the eyes with this habit ; for 

 it is clear that when so resting, if the eyes were 

 placed in the usual positions, one at each side of the 

 head, the eye on the paler surface — i.e., the surface on 

 which the fish lies — would not only be perfectly use- 

 less, but would be liable to injury from contact with 

 the sea-bottom. 



On turning to the development of the flat fish we 

 find this supposition confirmed. A sole on hatching, 

 and for some time afterwards, has its eyes one on 

 each side of the head, just like any ordinary fish ; 

 furthermore, it swims, like other fish, with the body 

 vertical, and has its two sides coloured alike. It is 

 only after it has attained some size that it gradually 

 adopts the habits of the adult, and takes to resting 

 on its left side on the sea-bottom and swimming with 

 the body horizontal instead of vertical. At the same 

 time, the right side of the body gradually becomes 

 coloured differently to the left, and in such a way as 

 to resemble the sea-bottom closely, and so enable the 



