294 VARIATIONS AND RELATIONSHIP OF 



These characters were the number of vertebra? in the caudal 

 peduncle and in the tail, the number of vertebra? which have no median 

 ha^mapophysis, and the total number of vertebra?; the number of liu 

 rays in the dorsal and anal fins ; the length of the caudal peduncle, 

 and its mean height ; the total length of the body without the 

 caudal fin ; the length of the head on the ocular side ; and the 

 number of branchiostegal rays. In order to eliminate small errors 

 of measurement, such as those which result from the shrinkage of 

 specimens which have been preserved in spirit, and to give at the same 

 time a clearer expression of the main facts of variation, the total range 

 of variation in respect of each particular character was legitimately 

 divided into a small number of arbitrarily selected divisions. Into one 

 of these divisions a number of individuals, all varying slightly from 

 one another, would then fall ; and the individuals in question would be 

 regarded as identical in respect of the particular character examined. 

 The author has, however, considered it necessary to adopt a notation to 

 represent the " variation degrees " of each character, which makes his 

 paper by no means easy to read. The several characters of particular 

 individuals are represented by formula? which the reader has to inter- 

 pret by reference to the chapter on the method employed, whenever 

 they occur. Thus we are told that the " Extreme Flounder form " has 

 characters represented by the formula 5 + 19 + 9 = 33. a a S a (1). 



Having explained his method, the author gives in his next chapter 

 a table, in which the limits of variation for each particular character in 

 the two species considered are indicated. 



In two forms so closely allied as the plaice and the flounder, a 

 considerable part of the entire range of variation for any character is 

 often common to the two species. 



The table given shows in an interesting manner the degree to which 

 the variation ranges of the several characters are distinct in the two 

 species. Thus the number of fin rays in the dorsal fin varies from 

 51-65 in the flounder, and from 61-80 in the plaice. 



The total range of variation is 30, and of these 30 possible variations 

 there are 5 which are common to both species. This fact is expressed 

 by saying that the percentage of variation common to the two species 

 is 17 per cent. It is noteworthy that in respect of the depth of the 

 body and the length of the head — the measurements in each case being 

 expressed as percentages of the body length — there is no difference in 

 the ranges of variation for the two species. 



Our author next deals with the infiuence of age and sex. This part 

 of the subject appears to us to be somewhat inadequately treated. In 

 dealing with local forms, the author states that he examined exclusively 

 "the grown up" specimens ("erwachsenen materials") without defining 



