RECAPITULATION 199 



degrees of the abnormality occur : in Turbot witli 

 the hook-Hke projection of the dorsal fin the lower 

 side of the head is often without pigment, while the 

 rest of the lower side is pigmented. Less degrees 

 of pigmentation of the lower side occur without 

 structural abnormality of the eye and dorsal fin. 



There is no evidence that these abnormalities are 

 due to abnormal conditions of life. One specimen 

 of Plaice of this type was kept alive in the aquarium, 

 and it lay on its side, buried itself in the sand, and 

 when disturbed swam horizontally, like a normal 

 specimen. The abnormalities are undoubtedly 

 mutations of gametic origin. The development of 

 one of these abnormal specimens from the egg has 

 not to my knowledge been traced, but there is no 

 reason to suppose that the fish develops first into the 

 normal asymmetrical condition and then changes 

 gradually to the abnormal condition described. On 

 the contrary, everything points to the conclusion 

 that the abnormality is an arrest or incomplete 

 occurrence of the normal process of development, i.e. 

 of the normal metamorphosis. T. H. Morgan, in a 

 volume published some years ago,^ put forward the 

 extraordinary view that the Pleuronectidae arose 

 from symmetrical fishes by a mutation which was 

 entirely gametogenetic and entirely independent of 

 habits or external conditions, and then finding itself 

 with two eyes on one side of its head, and no air- 

 bladder, adopted the new mode of life, the new habit 

 of lying on the ground on one side in order to make 

 better use of its asymmetrically placed eyes. Accord- 

 ing to this view habits have been adapted to 

 structure, not structure to habits. We arc thus to 



* Evolution and Adaptation. 



