212 



Part III. — Seventeenth Annual Report 



very little less than ± of the skull-length, and the skull-depth a little 

 more than J of the same dimension. The difference between the males 

 and females — viz., -7-:l — for the skull-length has to be multiplied by these 

 fractions in order to give the approximate amount of difference in the 

 breadth and depth relative to the body-length. The skull-breadth is 

 therefore relatively greater in the females by , 36°/ O0 than in the males, 

 and the skull-depth is similarly greater by , 25°/ 00 . 



The reason for this difference in the dimensions of the skull between 

 the males and females does not seem difficult to perceive. It is probably 

 connected with the position aud mass of the reproductive organs. These 

 are larger in the female and are placed further back than in the male. 

 The centre of gravity in the plaice seems to lie close to the first caudal 

 vertebra, and the male reproductive organs are placed along the haemal 

 and interhaBtnal spines of this vertebra, and anteriorly to it, whilst the 

 female organs lie along the interhremal spines posteriorly. The greater 

 size and therefore mass of the skull in the female would counterbalance 

 anteriorly the greater mass of the reproductive organs posteriorly. 



Tail. — Table XII., p. 231, shows that there is very little sex-variability 

 in the length of tail. In the females it is longer in four groups, but in the 

 fifth group the males have a slightly greater length of tail than the females. 

 It is therefore difficult to decide from the observed specimens whether this 

 variability is present in the tail or not. A slight probability tells in 

 favour of the females having the longer tails, and this may be correlated 

 with the differences in the body and skull already described. 



It should be mentioned that the length of tail was measured along the 

 longest fin-ray, the median one. The proximal end of the fin-ray is 

 found by bending the fin to the side, and then the point of junction of 

 the fin-ray with the ring of cartilage that terminates the hypural elements 

 is obtained with little difficulty. This dimension is perhaps of little value 

 on account of the variation in size caused by fraying of the distal 

 extremity. This affords another good reason for deducting the tail-length 

 from the total-length of the fish in choosing the standard dimension — in 

 percentage-values of which the other dimensions are expressed. 



II. Groirth-rariahility. — This form of variability is a much more 

 important factor in giving rise to differences between groups of individuals 

 than is sex-variability, and must be carefully eliminated before conclu- 

 sions can be made with respect to race-variability. It is, further, a much 

 more complex factor and gives rise to problems extremely difficult to 

 unravel. 



Some of these may be briefly referred to. There is, first, the possible 

 difference of age in specimens about the same size, and in the case of 

 some organs, as, for example, the skull, it might be thought that the 

 hardening process or ossification would depend upon age and not upon 

 growth. There seems no clue to the solution of this difficulty for the 

 present, but if it be considered that, so far as known, there is no definite 

 limit to the growth of such parts in fishes — unless we imagine such a limit 

 to be like the limit of a convergent series in mathemathics — and that 

 time or age is therefore of minor consideration, then attention may be 

 directed to the question of change in growth only. Treating the flatter 

 more definitely, if we suppose that of two fish about the same size one 

 may be seven years old and the other only five, then our conception of a 

 convergent series in mathematics permits us to disregard the difference in 

 age — because the difference in the size of a part considered as due to this 

 difference of age would be of the third or fourth order of infinitesimals in 

 comparison with the sizes of the parts dealt with — and thence to consider 

 the two specimens as of the same age. In actual observation, also, we are 



