Instincts, Habits, and Adaptations 177 



" The fact that the change in all fishes 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 flatfishes of A 

 species referable to genera living at present, AT 

 Rhombus (Bothus} and Solea, are found in // 

 the Eocene deposits. These two genera I 

 are notable in that Bothus is one of the ^JH 

 least and Solea the most unsymmetrical of 

 the PleuronectidcB. 



' ' The degree of asymmetry can be cor- 

 related with the habit of the animal. Those FlQ 132 _ Face view of 

 fishes, such as the sole and shore-dwelling recently hatched Floun- 

 flounders, which keep to the bottom are the der - (After s. R. Wil- 

 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 which 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 ad- 

 vantageous selection favored its appearing earlier and earlier 

 in ontogeny, until it occurred in the stages of planktonic life. 

 Metamorphosis at a stage earlier 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 metamorphosis by the removal of those individuals which 

 attempt the transformation at unsuitable epochs; for instance, 

 at the time of hatching. That there are such individuals is 

 shown by Fullarton, who figures a fish just hatched ' antici- 

 pating the twisting and subsequent unequal development ex- 

 hibited by the head of Pleuronectids.' Those larvae which 

 remain pelagic until better able to compete at the sea-bottom 



