REPORT OF COMMISSIONERS OF INLAND FISHERIES. 57 



Thus the Lamarckian explanations of the asymmetry of the flat- 

 fishes seem to be difficult to accept, since the necessary mechanical 

 requirements are not met by the bone and muscle arrangements of 

 the head, and because it is difficult to consider the "use and disuse" 

 theory as accounting satisfactorily for the evidences of asymmetry 

 in the germinal organization. The more fundamental difficulties 

 which are inherent in any Lamarckian hypothesis would, if it were 

 possible to consider them here, give still greater force to these objec- 

 tions. It is sufficient to say that the lack of evidence in support of 

 alleged cases of inheritance of acquired characters gives such force 

 to the a priori objections to that theory that, on general grounds 

 alone, we would be justified in rejecting Lamarckian explanations of 

 the origin of the modifications of the flat-fishes. 



A much more satisfactory hypothesis to account for the structural 

 modifications of the flat-fishes is that furnished by the theory of 

 natural selection. This theory as applied to the flat-fishes can be 

 accepted at the present time with much better grace than was 

 possible before the theory of mutation had been suggested. The 

 flat-fishes present a problem which is an excellent example of a class 

 of cases which have long been a bete noir to the theory of natural 

 selection. It has not been easy, previously, to understand how 

 slight tendencies of symmetrical fishes to assume other than the 

 upright position could confer upon their possessors such advantages 

 in the struggle for existence that natural selection could have been 

 an effective factor in causing their survival. But according to the 

 mutation theory we may assume that a considerable degree of the 

 characteristic modification of flat-fishes was produced all at once as a 

 result of some alteration in the structure of the germ ceil; this is 

 consistent with the facts of the case, since evidence adduced above 

 indicates that we are to look for the source of the origin of these 

 characters in the germ cell. These changes or mutations might be 

 great enough to make the new form differ from the old so far that it 

 was enabled to secure a somewhat greater advantage in the struggle 

 for existence by becoming more closely adapted to a bottom existence 



