of the Fishery Board for Scotland. 



215 



will make this distinction clear. In these, the specimens from two 

 different regions are arranged into three groups — immature specimens 

 between 250-350mm. in length, mature specimens of the same length, 

 and mature specimens over 350mm., mostly between 350-450mm. If 

 we consider any one of these groups, say, the immature, then the age of 

 the specimens is uncertain. The individuals between 250-350mm., 

 which are grouped together, may be three, four, or five years old, so far as 

 our present knowledge goes. But this difference for the reasons given 

 may be disregarded. All the groups are alike in this respect and thus 

 directly comparable. On the other hand, the average in each group is 

 the average of a range of variations which, even if we suppose the 

 individuals in each group to be of the same year, have arisen from the 

 initial differences due to age. Hence, in comparing the groups from the 

 two different regions — southerly North Sea and Aberdeen — it is necessary 

 that the probable fluctuations of the averages of the variations due to the 

 latter cause should not overlap if we are to find racial distinctions. 



The use of the words " differences due to age " in this sense may appear 

 at first sight a little confusing, but when understood the distinction is of 

 importance and the usage allowable. 



If, now, two groups of different sizes, but from the same region — e.g., the 

 mature specimens from the southerly North Sea — between 250-350rnm. and 

 350mm. and above, are compared, it is evident that the differences in the 

 averages are not due to any of the causes yet mentioned. Such differences 

 are usually ascribed to "growth-variability." In the length of the skull, 

 for example, the average size — i.e., the average ratio of the skull-length to 

 body-length — diminishes with growth. This means that at whatever rate 

 the body grows the head does not grow so fast. It follows from this 

 example that it might lead to quite erroneous conclusions if a group of 

 mature specimens between 350 and 450mm. coming from one region was 

 compared with a similar group, but between 250 and 350mm., coming from 

 another region. The differences found might really be due to this 

 growth-variability, although ascribed to race-variability. 



It is difficult to say what this growth- variability may mean. Separate 

 explanations have to be given for each character that presents this 

 variability, but in general it may be connected with the balancing of the 

 different organs and structures. The weight of the head, for example, 

 will increase in a greater ratio than will the length of the skull, and it 

 may be, therefore, that the greater rate of increase of the body-length 

 relative to the skull-length is correlated with the rate of increase of the 

 head-weight. 



In the discussion of the growth-variability of the various dimensions 

 the influence of maturity will also be mentioned. 



Vertebrce and Fin-rays. — In the following Table a collection of 341 

 plaice from Heligoland is divided up into three lots according to the 

 differences in size : — 



[Table, 



