of the Fishery Board for Scotland, 



285 



The changes that have arisen from race-variability, therefore, are just the 

 reverse of what have occurred through growth-variability. 



The differences that arise in the skull from sex-variability, again, have 

 been shown to be very slight, and tend to demonstrate that the skull is 

 relatively a little larger and heavier in the female than in the male. 

 These differences, however, do not in any way affect the evidence for race- 

 variability, because the differences arising from the latter cause appear 

 both in males and females. 



It appears, therefore, that changes arise in the skull from all three forms 

 of variability, and it has been sought to find the reason for these changes 

 in the inferred alterations in volume and mass or weight, and thence in 

 the maintenance of a continual balance between the head and body. 

 Under sex-variability it has been inferred that the greater mass and weight 

 of skull in the female is correlated to the greater mass and weight of the 

 reproductive organs. Under growth-variability it has been inferred that 

 the dimensional relative decrease in the skull accompanying the relatively 

 stationary condition of the body means that the mass and weight of the 

 skull has remained throughout in exact proportion to the mass and weight 

 of the body. Lastly, under race- variability, it has been inferred that 

 although the dimensional changes from north to south balance one 

 another, yet that the mass and weight of the head as a whole in reality 

 decreases. This may be correlated with the changes that occur in the body. 



Body. — The changes that occur in this dimension arise solely from race- 

 variability. Nor was it to be expected that changes would arise here 

 from sex and growth variability. If any difference arose in the body from 

 these causes it would be between the dorsal and ventral halves and not in 

 the body-height as a whole. It may be inferred also that the differences 

 lie not only in the greatest body-height, but along the whole length of the 

 body from head to tail. Any great increase in weight, therefore, in the 

 region of the body posterior to the first caudal vertebras would be partially 

 counterbalanced by the greater weight of the abdominal region. 



If we now consider the body-height and skull together we may conclude 

 that the changes in the latter arising from sex and growth-variability are 

 correlated with the ripening of the reproductive organs, but that the 

 changes in it arising from race-variability are correlated mostly with the 

 changes in body-height. It has been shown that the northerly plaice have 

 broader and deeper skulls than the southerly, and this is associated with a 

 greater body-height. In the northerly plaice the skull is larger posteriorly 

 than in the southerly, and this is probably related to a greater develop- 

 ment of the opercular and pectoral arches, and of the muscular system 

 round the skull. And it is probable, again, that this greater development 

 anteriorly aids to counterbalance in mass and weight the greater mass and 

 weight of the posterior caudal region in spite of the lesser length of the 

 skull. If, therefore, we regard the balance of organs in the northerly and 

 southerly groups to be still the same, we may picture the difference as one 

 of form. The northerly plaice are rounder fish than the southerly. The 

 body of the plaice has very approximately the form of an ellipse, and the 

 change that has occurred in passing from north to south shows itself in 

 the relative decrease of the minor axis (body-height) and relative increase 

 of the major axis (total length of fish). 



If we consider, as is generally believed, that the plaice is descended 

 from a northerly sub-arctic stock, then the change above described has 

 probably been the change that has actually occurred. And this alteration 

 has been the only one — no change, so far as can yet be detected, taking 

 place in the vertebrae and fin-rays. The manner by which the change has 

 actually occurred can only be conjectured. It may be that the earlier or 

 younger condition has become the older condition in the southeriy plaice, 

 p 



