368 A HISTORY OF FISHES 



different forms of Char (Salvelinus) found in the lakes of Switzer- 

 land, Scandinavia, and the British Isles, are probably to be 

 looked upon as sub-species of the widely distributed Alpine 

 Char {S. alpinus) , which is a migratory fish in the Arctic Ocean. 

 Here again, however, it must be remembered that some 

 systematists would regard many of the lacustrine Char as 

 distinct species, the only drawback to such a view being that 

 once one starts giving specific names to such forms, there 

 is no knowing where the matter will end. To turn to another 

 example, recent research has shown that several of our important 

 food-fishes, such as the Herring (Clupea harengus) and Plaice 

 {Pleuronectes platessa) can each be split up by experts into a 

 number of races, each with its own slight morphological 

 peculiarities, area of distribution, and time and place of 

 breeding. The differences between the races are sometimes so 

 slight that the different forms can only be distinguished when 

 a large number of examples are examined. Such races have 

 not yet received sub-specific names, but they are of exactly 

 the same nature as the lacustrine Char, and it may be concluded 

 that in general the terms "race" and "sub-species" mean one 

 and the same thing, the differences between them being only 

 of degree. 



It sometimes happens that certain individuals of a species 

 differ from the normal or mean to a greater or lesser extent, but 

 such differences are not found among a particular community, 

 nor are they related in any definite way to the habits of the 

 fish or to its environment. For example, among the individuals 

 of a species characterised by its uniform coloration there may 

 be some which exhibit a black spot on the head or a series of 

 dark bars on the sides of the body. The name varieties may 

 be given to such individuals, although this term has been used 

 by some authors in a totally different sense. Well-known 

 examples of varieties are the Gold-fish, Golden Carp, Mirror 

 Carp, Leather Carp, Golden Trout, and so on. 



So much for the lower units of classification. It is not 

 sufficient, however, to group the individuals together into 

 species, but the species must in their turn be arranged to form 

 genera. Again, there can be no hard and fast rule as to the 

 morphological differences necessary to distinguish one genus 

 from another, it being entirely a matter of convenience and 

 indi\'idual opinion; and it will sometimes happen that one 

 worker will split up into a number of genera a group of species 

 which another systematist would assign to a single genus. 



