of the Fishery Board for Scotland. 



239 



We may turn now to the theoretical consideration of the conclusions 

 that the plaice as a species is divided naturally into several groups, and 

 we may endeavour to answer the question how these groups have arisen. 

 We may believe that they have sprung from a more primitive form which 

 inhabited the sub-arctic regions. Migrating southwards, they have 

 gradually spread out over the comparatively shallow areas of the temperate 

 zone, penetrating to wherever the conditions were suitable to their habits 

 of life, and as they reached the successive regions their characters slowly 

 altered. 



Such seems to have been the progress of events, but we do not 

 know how the changes have been wrought. In the case of the Baltic 

 groups the evidence seems almost conclusive that the direct action of the 

 changed environment has effected the changes in vertebrae and fin-rays. 

 These are formed in the early post-larval stages, and in some way that 

 we do not yet understand the decrease in salinity and the increase of 

 temperature in the surrounding waters have brought about a decrease in 

 the number of vertebrae and fin-rays. Correlated with this decrease the 

 average size of the adults has also decreased, and the relative proportions 

 of the various organs and structures have altered in the same directions 

 as those of the southerly North Sea plaice. 



In the case of the North Sea plaice we find that there is no appreciable 

 change in the number of vertebrae and fin-rays, and we may thence 

 conclude that the range of variations and the averages of the salinity and 

 temperatures over the whole North Sea are approximately the same 

 during the periods that these organs are developing. This belief is 

 strengthened when Ave find that the plaice of the south-west coasts of 

 Scotland* possess the same average number of vertebrae and fin-rays as 

 those of the North Sea. These probably come under very similar 

 conditions of salinity, and possibly also of temperature. 



It is evident that careful records of temperatures and salinities over 

 these different areas are much needed for such a work as this, where fine 

 differences in characters are sought for. 



On the other hand, it has been shown that the North Sea plaice are 

 divided into at least two groups by definite and distinct differences in 

 structure. These differences show themselves in alteration of proportions, 

 such that the plaice of the southerly North Sea are relatively more 

 elongated and narrower, where those of the northerly North Sea are 

 shorter but broader. The plaice of the latter region are more massive, 

 where those of the former are more slender. It has been shown that in 

 all probability this change has taken place without any alteration of the 

 balance of the various organs — i.e., the centre of mass or balance of the 

 animal remains the same. This has been shown to be the o?.se also with 

 the relative changes in parts which arise from sex- and growth-variability. 

 If we suppose that this balance is constant for the species, then the 

 alterations in the relative proportions of structures due to sex, growth, or 

 race are evidences of the various possible combinations of structures and 

 organs which may arise from the specific variability of the organism. 



* Dr. Fulton informs me that the salinity of the Solway Firth is certainly lower than 

 that of the North Sea, though actual statistics are not to hand. I he plaice from the 

 Solway are therefore comparable rather with those from the Helder than with those of 

 the North Sea, because those from Helder are reared most probably in the brackish waters 

 of the Zuyder See. The values of the averages for these two regions [vide Tables VI., 

 VII., and IX.), with the exception of those for the anal and right pectoral fins, are also 

 more akin than with those for the North Sea. It maybe, as already suggested (p. 77 and 

 elsewhere), that plaice from both these regions are distinct races, i.e., distinct from those 

 of the North Sea, but other characters than the fin-rays must be more closely examined. 

 The evidence given in this paper shows that for' the plaice the averages of the 

 vertebra? and of the fin-rays do not materially alter except under very marked 

 differences in salinity, as, for example, in the case of the plaice in the Baltic. 



