RECAPITULATION 217 



eye muscles. The formation of the bony bridge 

 outside the dislocated eye is more difficult to explain, 

 as I have never had the opportunity to study the 

 relation of this bridge to the muscles. It is worth 

 mentioning that in the actual development of 

 . Turbot and Brill the metamorphosis takes place to a 

 considerable degree while the young fish is pelagic, 

 before the habit of lying on the ground is assumed, 

 but of course this is no evidence that the change 

 was not originally caused by the habit of lying on 

 the ground. 



With regard to the extension of the dorsal fin there 

 is no difficulty in discovering a stimulus which would 

 account for it. Symmetrical fishes propel them- 

 selves chiefly by the tail ; in shiffling over the 

 ground or swimming a little above it, Flat-fishes 

 move by means of undulations of the dorsal and 

 ventral fins. Increased movement produces hyper- 

 trophy, and according to the theory here maintained, 

 not merely enlargement of parts existing, but 

 phylogenetic increase in the number of such parts, 

 here fin rays and their muscles. In Flat-fishes the 

 dorsal and ventral fins extend along the whole 

 length of the dorsal and ventral edges : the dorsal 

 from the head, in some cases from a point anterior 

 to the eyes, to the base of the tail, the ventral from 

 the anus, which is pushed very far forward, to the 

 base of the tail, and in some species of Solidae these 

 fins are confluent with the caudal fin. 



Formerly it was dogmatically maintained that 

 the effect of an external stimulus on somatic organs 

 or tissues could have no influence on the determinants 

 in the chromosomes of the gametes to whicli the 

 hereditary characters of the organism were due. 



