502 SCIENCE PROGRESS. 



in the adult flat fish, and dermal spines somewhere on the 

 head. These dermal spines are additional evidence of the 

 want of affinity between the Pleuronectidai and Gadidse. 

 In searching for the possible ancestors of the flat fishes 

 among extinct forms, we find the required characters most 

 clearly represented in the Pycnodonts, a compact group 

 which ranged from the Lias to the Eocene, in which latter 

 formation fossil flat-fishes first appear. The Pycnodonts 

 have a single dorsal and single anal fin, and each of these 

 in some genera is much extended antero-posteriorly. The 

 body also is compressed laterally and rhomboidal in shape. 

 But it is doubtful whether the specialised dentition of the 

 Pycnodonts could have given rise to the dentition of the 

 Pleuronectidse. 



The precise nature of the changes which have taken 

 place in the flat-fish in its modification from a symmetrical 

 fish is as follows. The orbital region has been twisted so 

 that the interorbital septum is brought round to the upper 

 side, and the eye of the lower side to the upper. It cannot 

 be said that we can point out with certainty the muscular 

 efforts or mechanical strains which would produce changes 

 in this direction in the individual as an acquired character. 

 The action of the eye-muscles would probably have some 

 effect, and the weight of the fish resting on the ground 

 would force the lower eye-ball towards the upper side and so 

 distort the face. In all probability both these influences 

 have contributed to the result. If acquired characters are 

 not inherited, at least evidence and logic force us, whether 

 we will or not, to the conclusion that the modifications which 

 have led to the condition of the eyes in flat fishes were de- 

 termined by the mode of life which these fishes adopted. 

 There is no evidence known to me which supports the view 

 that such modifications, or variations which could lead by 

 accumulation to such modifications, could possibly arise 

 independently of the influence of external conditions. 



Another modification which has occurred is the ex- 

 tension of both the dorsal and ventral fins in an anterior 

 direction. The ventral fin is extended towards the head, 

 pushing the anus before it, and passing ventral to the 



