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 litle 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 
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 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 
