Bilateralism. 153 



ing parts of the brain, viz. , the optic lobe on the upper side and the 

 hypoaria on the lower, are much larger than their fellows which are in 

 connection with the right eye. 



The flat fishes are destitute of an air bladder so that they remain near 

 the bottom without effort. They are sly and cunning and quickly con- 

 ceal themselves by burrowing in the sand or mud on the bottom, so that 

 their eyes alone remain uncovered. Both the loss of the bladder and 

 the acquisition of the cunning qualities are due to the habits that their 

 * ' pursuit of happiness " impelled them to adopt. Finding their best 

 supply of food near the bottom they seldom needed to inflate their blad- 

 der, and so it became atrophied. Concealment in the mud not only af- 

 forded security for themselves, but deceived the caution of their in- 

 tended prey. Their human adversaries, however, set their wit at 

 naught by plowing for them with a sort of hooked tool, like a long 

 handled sickle. But when they lay there concealed by mud and the 

 dark pigment which covers their upper side, if any small fish conies in- 

 cautiously close enough they will dart out and seize it. To watch for 

 their prey they need all their eyes, and so the one which would have 

 been buried in the mud on the underside, has been resurrected and 

 brought to the top. It is easy to imagine how, generation after genera- 

 tion, this twisting process would go on pari passu with the constantly 

 changing position of the fish from his original perpendicular, in his ef- 

 forts to get nearer the bottom ; and how the pigments in his skin origin- 

 ally, no doubt, on his back and half way down his sides, as in the 

 Perch, Salmon, &c. , shifted over with his eyes to occupy the top side, 

 selection interfering constantly to preserve for survival those best con- 

 cealed, that is, those in which the deformity had made the most progress 

 and which were able to lie the flattest on the bottom. Of the flat fishes 

 ( or Pleuronectidae ) the Plaice, Flounder, Dab, Laminder, Halibut, 

 Sole, Monochirus and Achirus are Rights, that is, have the right side 

 up; while the Turbot, Brill, Topknot, Whiff, Scarlet fish, &c., are Lefts. 



I conjecture that both the "doubles" and the reverted individuals 

 may be hybrids or crosses. 1 , 



The question might be asked why or how such a distinction as that 

 between the right hand and the left could occur ; why the one should 

 take the lead and the other always accept a subordinate position. At 

 first sight it would seem that the external stimulus ought to act alike on 

 each and that one would naturally be as dextrous as the other. But 

 reflection shows it to be impossible. Suppose the external stimulus 

 resolved, self-co-ordinated and equilibrated in the balances of the brain, 

 requires the driving of a nail. It is a single stimulus but it is made up 



1 An article in Pop. Sci. Magazine, Vol. 27, published since the above was written.takes 

 much the same view of the flat fish as that given here, and adds that the flat fish is de- 

 rived from the Cod family. 



