238 NATURAL SCIENCE [October 



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 metamorphosis 

 of other species of flat-fish are similar. The early condition of the 

 fiat-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 an- 

 cestor 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, and we have reason to believe they 

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

 by gradual increase. The true reading of the matter therefore is„ 

 not that the ancestral condition is repeated, but that the larval 

 condition of the ancestor is retained, because the larva is still 

 hatched and still lives in the same way ; but the structure after 

 metamorphosis is different because the adult fish has acquired differ- 

 ent habits. On the theory of natural selection we must suppose 

 that those individuals have been selected whose eyes were most 

 symmetrical in the larval stage, and most asymmetrical in the adult 

 condition. But we have no evidence that among symmetrical fishes 

 individuals occasionally occur in which one eye moves up towards 

 the edge of the head during growth. Even if slight variations of 

 such a character were proved to exist, it would be difficult to believe 

 that they would be great enough to make any difference to the fate 

 of the individuals possessing them when the fish took to lying on the 

 ground. The theory of independent variation and selection as 

 applied to flat-fishes is unsupported by evidence, while the conclusion 

 that the metamorphosis of these fishes is the direct result of the 

 change of conditions is in harmony with all that we know of the 

 effect of physical conditions on individual organisms. 



In these two cases, that of the frog and that of the flat-fish, the 

 larval condition is either unmodified or less modified from the ances- 

 tral condition than the adult. But in numerous other cases the 

 larva has been modified in adaptation to new conditions while the 

 adult has remained nearly the same. This is particularly conspicu- 

 ous in many insects. I will not discuss at length the question of 



