of flic Fisher?/ Board for Scotland. 



213 



obliged to follow this course, because we cannot as yet fix the age of a fish 

 by its size beyond the first year or two. 



In connection with this question an interesting paper by Reibisch* 

 which has recently been published may be referred to. He endeavours 

 to show how it is possible to tell the age of a plaice by an examination 

 of the otoliths. These, according to his view, grow by successive deposi- 

 tions of calcareous matter, and hence the rings formed at the successive 

 stages may be utilised to tell the age of the fish, just as the successive 

 rings in wood tell the age of a tree. This interesting suggestion came too 

 late for use in the observations recorded in the present paper, but it will 

 not be difficult to act upon it in future. 



A further problem arises when the rate of variation in fishes of presum- 

 ably the same age is considered. If a group of individuals be taken 

 which are alike in the condition of their reproductive organs — e.g., 

 immature males, and which are nearly alike in size — e.g., between 250 and 

 350mm. — it is found that they differ greatly from one another in every 

 character, and these differences when arranged according to the method 

 of variations give a certain average condition and a certain range of 

 variation round this average. The average condition may be compared 

 directly with the average of other similar chosen groups, but the question 

 at present is to account for the range of variations. With respect to 

 certain characters — e.g., numbers of vertebra? and fin-rays — which, as will be 

 shown, do not vary with growth, the range of variations must arise from 

 influences which are operative on the organism when these parts are in 

 process of formation. These parts are fixed at the earliest stages, when 

 the fish is in the larval and post-larval condition, and the influences that 

 affect their formation are for the most part the surrounding physical and 

 chemical conditions. It might be thought, and it has been stated, that the 

 differences in the amount of food — i.e., of nutrition — may affect the forma- 

 tion of these parts, but it seems to me that such a factor is probably of 

 secondary importance. These parts are not formed in a few minutes, so 

 that the momentary condition of the animal has to be taken into account, 

 but in the course of some days and even weeks. Considering, therefore, 

 that the chances of obtaining food are the same for individuals hatched 

 about the same time, it is possible that the differences in the physical 

 and chemical conditions are of the first importance, and the differences in 

 nutrition only secondary, in causing the variations of these parts. 



This question may be left open, however, until further observation and 

 experiment provide more satisfactory data. Under artificial conditions it 

 is probable that the differences in nutrition are the sole cause of differences 

 in growth, t but whether, under natural conditions, such differences in 

 the amount of food obtained by each fish can arise may be doubted. 



On the other hand, the differences in the physical and chemical 

 conditions when these parts are in process of 'formation are well 

 marked. The spawning-period for the plaice extends over three months, 

 from January to March, for the most part, and the average change 

 of the external conditions during those months is considerable, though not 

 yet known very definitely. Hence these differences in the external 

 conditions are perhaps of chief importance in causing the variation in the 

 parts mentioned. It will be said, however, that the organisms which 

 provide food for the young fish are the more abundant the further the 

 season advances, and hence that the difference in nutrition may play an 



* Reibisch, T. : "Ueber die Eizahl bei Pleuronectes platessaund die Altersbestimmung 

 dieser Form aus den Otolithen," Wissen. Meercsunter. Abtheilung, Kiel, B. IV., 1899. 



f Vide Cunningham, J. C. : "The Rate of Growth of some Sea-fishes, etc.," Jour. 

 M.B.A., v. II., 1891-92. 



