Evolution 



There are four main divisions of the fish group. The first, 

 that of the Elasmobranchs, comprises the sharks and dog-fishes, 

 the skates and the rays. The second group, the Ganoids, includes 

 the sturgeon and a few less well-known forms. The third, the 

 so-called bony or food fishes, includes the vast majority of ordinary 

 species, such as the salmon and trout, the cod, herring, eel, and 

 all our ordinary freshwater species. The fourth, the ' lung 

 fishes/ consists of three very remarkable species, which we shall 

 later describe in detail. The mutual relationships of these groups 

 is well understood, and it is possible to make fairly definite state- 

 ments regarding their evolution. 



The Elasmobranchs are at once the most primitive and, 

 so far as is known, the oldest of the four. From these evolved 

 the lower Ganoids, which then divided into two main branches, 

 the first of which led up to the higher Ganoids and through them, 

 at a comparatively late date, to the bony fishes. The second 

 led to the lung fishes and, either through them or along a some- 

 what parallel line, to the amphibians and the land vertebrates 

 generally. It is with the second line, therefore, that we shall 

 be mainly concerned. 



The Elasmobranchs are characterised by the fact that the gill 

 slits open individually to the exterior, there being no gill cover, 

 such as is found in the other groups. Their scales are simple, 

 tooth-like projections, and in fact there is no essential difference 

 between them and the teeth. The skull is more primitive than in 

 the other groups, but a discussion of its details would necessarily 

 be very involved. The living members of the group show a fairly 

 high stage of development of the vertebrae — considerably higher, 

 in fact, than that found in the lung fishes— but some extinct 

 members showed a very primitive condition with regard to this 

 point. In the fossil skeleton shown in Fig. 71, for instance, it 

 is apparent that the notochord was present as a simple continuous 

 rod. The skeleton in question is from the Permian and belongs 

 to what is regarded as the most primitive type of fish known. 

 Two specimens of Elasmobranchs are shown in Figs. 72 and 73, 

 and the teeth of a shark in Fig. 74. 



94 



