,3,,. THE EVOLUTION OF FLAT-FISHES. 193 



symmetrical ancestors, and that their individual metamorphosis is a 

 recapitulation of their secular evolution, is now a mere truism. We 

 have to consider the causes of the evolution and the causes of the 

 metamorphosis. 



There are doubtless some whose minds are satisfied with the 

 purely orthodox Darwinian explanation. The theory is so simple and 

 so universal, and it is not only so much easier to believe than to doubt, 

 but it saves so much trouble to have one key to all cases of evo- 

 lution, instead of having to find a special key for each by laborious 

 research and experiment. What can be more obvious than that the 

 peculiarities of the flat-fish are beautiful adaptations to its peculiar 

 mode of life ? It is certain that variations of the most diverse kinds 

 occur ; there are the variations of these very peculiarities in the flat- 

 fishes to prove it. What other explanation is required than the 

 survival of the most adapted individuals in each succeeding genera- 

 tion while the less adapted were continually slain in the never-ending 

 battle of Hfe ? 



Well, perhaps the weakest point in this argument lies in the 

 fundamental assumption that all the peculiarities of the flat-fish are 

 adaptations. Weismann says " alles ist angepasst," There must be 

 no quibbling here as to the meaning of adaptation or Anpassung. 

 The Darwinian theory depends for its force on the assumption that 

 every character of an organism is or has been of some definite 

 advantage to it in its endeavours to secure its own existence or to 

 propagate its species. What then is the advantage to the flat-fish 

 of the absence of chromatophores from the side on which it rests ? 

 No naturalist has ever ventured to suggest that it uses the 

 conspicuous whiteness of its lower side in courtship or in obtaining 

 food, and it is certain that when alarmed every flat-fish hides this 

 lower surface completely, burying itself in the soft ground on which 

 it lives. The Darwinian may reply that no positive advantage need 

 in this case be assumed, that the chromatophores have a special 

 function in concealing the fish by changing its tint, and that being 

 useless on the lower side they have disappeared from absence of 

 selection and economy of organisation. Why then have not the 

 iridocytes and the argenteum also disappeared, since these have much 

 to do with the coloration of the upper side ? 



This question of the absence of colour, that is, of chromatophores, 

 from the lower side of flat-fishes has given rise to the most curious 

 misconceptions. The observations of Alexander Agassiz, made only 

 fourteen or fifteen years ago, on the metamorphosis of these fishes, 

 constituted an epoch in the history of our knowledge of the subject, 

 for they reconciled the apparently contradictory conclusions of 

 Steenstrup and Malin concerning the migration of the lower eye. 

 But in the same paper in which Agassiz described the transformation 

 followed by him for the 'first time in living specimens, he has 

 hopelessly confused the two entirely distinct questions of the changes 



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