292 



Part III. — Thirteenth Annual Report 



must be based either upon conjecture or else upon his experiments 

 narrated above. As Petersen quaintly remarks, his methods are 

 * dangerous ! ' 



In natural conditions, especially in a gregarious fish such as the plaice, 

 the environmental factors of temperature, nutrition, &c, must be very 

 closely similar, if not identical, for all the individuals of one brood, so 

 that, taking this into account, and also what has been stated about the 

 variation, there are good grounds for supposing that the rate of growth is 

 closely similar for each individual, and that a mean average length for 

 any given species at a given age is a very fixed and determinable quantity. 

 The greater the difference in environment the greater the difference in the 

 mean average, till ' forms ' may become marked. 



Coming to the facts, however, we find that, in any sample of a number 

 of young fry caught at the same time, they form a marked gradation 

 in size. These fry are caught in close proximity, and their environmental 

 influences being practically alike, their differences in size must be explained 

 by inherent tendencies and a diverse rate of growth, or else by a difference 

 in age. The latter is, I think, the true explanation of the facts, the 

 difference in age being directly traceable to the prolonged spawning period. 

 The facts with regard to the plaice are these : — A prominent feature of 

 the fauna of St Andrews Bay is the great abundance of young pleuro- 

 nectids, especially plaice, in the sandy shallows, and the specimens caught 

 in any particular month show a marked gradation in size. 



Thus in the month of September a series may usually be obtained, 

 varying in length between 3 J inch and 1^ inch. Two instances will 

 suffice : — 



1. September 11, 1889. — Mouth of the River Eden. 



Inches. 



Inches. 



Inches. 



Inches. 



3 



2-5 (2) 



2 



1-5 



2-75 (2) 



2-4 



1-95 



1-41 



2-7 



2-39 



1-92 



1-37 (2) 



2-65 



2*35 



1-81 









1-63 









1-62 









1-53 





2. September 15, 1889. — Near St Martin's Point. 



Inches. 



Inches. 



Inches. 



Inches. 



3-4 



2-9 (4) 



2-5 



2 



3-26 



2-85 



2-4 



1-95 



3-2 



2-65 (2) 



2-35 



1-92 





2-5 



2-3 (2) 



1-9 (3) 





2-55 



2-25 



1-85 







2-2 (4) 



1-82 







2T5 (3) 



1-8 







2-1 (5) 



1-55 







2 05 



1-6 



