22 SEXUAL DIMOEPHISM 



there can be no excuse for throwing doubt upon the accepted 

 doctrine that the larva of these fish, swimming upright in 

 the water with an eye on each side of its head, repeats in 

 individual development the condition of the ancestor. But 

 a more careful study of the facts shows that this doctrine is 

 erroneous, or at least only partially true, and it must be 

 modified to agree with the state of knowledge at the present 

 time. A brief summary of the facts will be sufficient to 

 prove this. 



The flounder, when first hatched, is a minute larva not 

 quite one-eighth of an inch in length. The right and left 

 sides are perfectly similar to one another, and it swims 

 vertically in the water. But it has no fin -rays, and no 

 bones : a continuous fin-membrane passes along the edge of 

 the back round the end of the tail. The conversion of this 

 larval form into the fully developed flounder takes place 

 when it is from two to three months old, and about half an 

 inch long. When the bones and fin-rays begin to develop 

 the left eye rises first to the edge of the head, and then 

 passes completely over to the right side. At the same time, 

 the little fish begins to lie on its side on the ground, and 

 loses the power of sustaining itself in the water. With 

 slight differences in details, the development and metamor- 

 phosis of other species of flat-fish are similar. The early 

 condition of the flat-fish, therefore, is not that of any fully- 

 developed fish at all, but of a fish larva without bones or 

 fin-rays. It is in all respects similar to the larvae of other 

 marine fishes ; for instance, to that of the mackerel or that of 

 the cod. When the bones begin to develop, the eye begins 

 to become asymmetrical, and we have not the ancestor, but 

 the flat-fish. We do not know at present whether the 

 elongated fins along the dorsal and ventral edges had the 

 same form in the ancestor ; we have reason to believe they 

 had not so great an extent, yet they are developed directly, 



