48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The “why” of the peculiar metamorphosis of the Pleuronectide is an 
unsolved problem. The presence or absence of a swim bladder can have 
nothing to do with the change of habit of the young flatfish, for P. 
americanus must lose its air-bladder before metamorphosis begins, since 
sections showed no evidence of it, whereas in Bothus the air-sac can 
often be seen by the naked eye up to the time when the fish assumes 
the adult coloration, and long after it has assumed the adult form. 
Cunningham (’92-97) has suggested that the weight of the fish acting 
upon the lower eye after the turning would press it towards the upper 
side out of the way. But in all probability the planktonic larva rests 
on the sea bottom little if at all before metamorphosing. Those taken 
by me into the laboratory showed in resting no preference for either side 
until the eye was near the mid-line. 
That the change in all species is repeated during the development of 
each individual fish, has been used to support the proposition that the 
flatfishes as a family are a comparatively recent product. They are, on 
the other hand, comparatively ancient. According to Zittel (87-90, pp. 
315-316) flatfishes of species referable to genera living at present, 
Rhombus and Solea, are found in the Eocene deposits. These two 
genera are notable in that Rhombus is the least and Solea the most 
unsymmetrical of the Pleuronectide. 
The degree of asymmetry can be correlated with the habit of the ani- 
mal. Those fishes, such as the sole and the shore-dwelling flounders, 
which keep to the bottom, are the most twisted representatives of the 
family, while the more freely swimming forms, like the sand-dab, summer 
flounder and halibut, are more nearly symmetrical. Asymmetry must 
be of more advantage to those fishes which grub in the mud for their 
food than to those which capture other fishes; of the latter, those that 
move with the greatest freedom are the most symmetrical. 
This deviation from the bilateral condition must have come about 
either as a “sport,” or by gradual modification of the adults. If by the 
latter method, —the change proving to be advantageous, — selection 
favored its appearing earlier and earlier in ontogeny, until it occurred in 
the stages of planktonic life. Metamorphosis at an age younger than this 
would be a distinct disadvantage, because of the lack of the customary 
planktonic food at the sea-bottom. At present some forms of selection 
are probably continually at work fixing the limit of the period of meta- 
morphosis by the removal of those individuals which attempt the trans- 
formation at unsuitable epochs, — for instance, at the time of hatching. 
That there are such individuals is shown by Fullarton (’91), who figures 
