CHAPTER VII 



VARIATION 



Continuous variations, their mathematical investigation, and relation to 

 locality. Discontinuous variations or mutations, e.g. in flat-fishes. Variation 

 under domestication. 



IT is well known that no two individual animals are exactly 

 alike, and the small differences which are always present 

 between individuals are technically known as continuous 

 variations, as when a large number of specimens are examined 

 they can be arranged in a gradual series of steps below and 

 above the degree of the character which is most frequent. 

 On the other hand, there occur occasionally conspicuous ab- 

 normalities which are not known to be connected by inter- 

 mediate stages with the typical form, and these are called 

 discontinuous variations. Latterly the term mutations has 

 been frequently applied to such variations. 



Continuous, i.e. small individual variations, have been studied 

 in some of the common British fishes of greatest commercial 

 value with great minuteness, for example in the plaice. 



In this species, as in most fishes, they are well exhibited by 

 the number of the fin-rays ; as in this species there is some dif- 

 ference between the sexes, it is necessary to treat the males 

 and females separately. 



The character and extent of such variations are most satis- 

 factorily exhibited in the form of a curve or diagram, such as 

 that shown in Fig. 29, which represents the numbers of dorsal 

 rays observed in a sample of North Sea plaice ; the total 

 number of fish in the sample, all females, was 91. The figures 

 at the base of the diagram are the numbers of rays, those at the 

 left the numbers of fish in which the different numbers of 

 rays occurred ; thus, it is shown that the most frequent number 

 was 72, which occurred in fifteen fish, while the numbers 67 

 and 79 occurred in only one specimen in each case. Other 



359 



