THE DEVELOPMENT OF VERTEBRATA. 289 



cases of adaptation, and it is a continuous process which 

 has no resemblance to discontinuous variation and is not 

 satisfactorily explained by any theory except that of the 

 heredity of modifications due to external conditions. There 

 is no evidence of variations which could possibly lead to 

 lungs, except in animals that breathe air. Similarly there 

 is no evidence that steps in the changes which take place 

 in the eyes of flat-fishes have ever occurred except in those 

 fishes which lie on one side. It is natural to conclude, 

 therefore, that the habits and conditions produced the 

 adaptations. There is plenty of evidence that such changes 

 of structure as are required on this theory are produced 

 by external conditions in the individual. 



14. The Metamorphosis of Flat-fishes. 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 the larva 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 metamorphosis of other species of flat-fishes 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 its 

 principal features 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 



