40 MAKKKTAIILE BRITISH MARINE FISHES 



the individuals of which do not require further distinction. It 

 is not necessary or possible to pay much attention to the trifling 

 details in which one herring differs from another, or one 

 mackerel from another, or one skate from another. It is 

 sufficient to speak of so many mackerel or so many 

 skate, or of a mackerel or skate of such a weight and 

 such a size. \Ye know that as an almost invariable rule 

 fishes, like other animals, exist in these distinct kinds which 

 are always recognisable, and capable of being distinguished from 

 one another. In each of the divisions which have already been 

 defined there are a great number of these distinct kinds or 

 species. But within each division again there are subordinate 

 groups of species, and in following out the history of the various 

 species it is necessary to have some acquaintance with these 

 further and subordinate groups. The species in fact are not 

 separated from one another by differences of equal degree, but 

 form natural families, each of which includes a number of species 

 distinct enough from one another, but having a family resem- 

 blance. Thus the pilchard, herring, and sprat are much more 

 like one another than they are to the species of another family, 

 such as the cod, haddock, and whiting. Again, even within the 

 family all the species are not equally different from one another, 

 but form still smaller natural groups, which the naturalist calls 

 genera, For example, the cod, haddock, and ling all belong to 

 one family, which we may speak of as the cod family, but 

 the cod and haddock are more similar to one another than 

 either is to the ling. Similarly, the herring and pilchard are 

 more like one another than either is to the anchovy, but all three 

 are placed in one family, called the herring family. 



We have now to take a survey of the principal kinds of fish 

 in the divisions which have been defined above, and to notice 

 the chief peculiarities by which they are distinguished from one 

 another. 



Our common dog-fishes on the one hand, and skates and 

 rays on the other, represent the two distinct types which are 

 found in the gristly or cartilaginous fishes. Dog-fishes and 

 sharks all belong to one great type or division, the term dog- 

 fishes being applied merely to the smaller and commoner kinds. 

 A great many kinds occur on our coasts, but none of them arc 

 of any considerable commercial value, although some are in 



