EVOLUTION AND PALEONTOLOGY 261 



viving forms, Lepidosteus and Amia of N. America, live in fresh 

 water, and the sea is inhabited by the latest results of evolution 

 in fishes, the Teleostei, together with Elasmobranchs or fishes 

 of the shark type. 



In numbers of individuals certainly, and probably in the 

 number of species also, fishes at the present time are superior to 

 all other classes of vertebrates, and this is not to be wondered 

 at when we remember that the sea covers about three-fourths 

 of the surface of the globe and that the waters of the land also 

 teem with fishes. Birds are able to range over the ocean as 

 well as over the land, but they are dependent on the land for 

 breeding. At present about 12,000 species of fish are known 

 and of these about 11,500 belong to the order of Tel eosteans. 

 The next most abundant order is that of the Elasmobranchs. 

 The Holocephali or Chimasroids are few in number and confined 

 to the deeper parts of the sea ; of Dipnoi Fringe-finned, and Bony 

 Ganoids there are only a few surviving species, and the Sturgeons 

 are not much more numerous, although some single species 

 among the last two groups are abundant in individuals, for 

 example Lepidosteus and Acipenser in North America. In 

 former periods of the earth's history the proportions were very 

 different : Teleostei began their domination in the Cretaceous 

 period, in the Jurassic or middle of the Secondary period the 

 Holostei were the most abundant, in the Trias and Carbonifer- 

 ous rocks remains of Chondrostei are most numerous together 

 with Crossopterygians and Elasmobranchs. Crossopterygians 

 and Dipnoi are characteristic of the Carboniferous and Devonian, 

 while the Elasmobranchs, which occur in all formations extend 

 back to Silurian times, where they are accompanied by the 

 strange forms called Ostracoderms, whose relation to the fishes 

 is as we have seen quite problematical. Fishes are dependent 

 on nothing but water and food, and wherever there is water there 

 are fish, some species being able to live at least temporarily even 

 where there is no water, as the eel and the tropical fishes pro- 

 vided with organs for atmospheric respiration, such as the climb- 

 ing perch (Anabas) and the lung-fishes (Dipnoi). The abund- 

 ance of some species where they have not been diminished by 

 human agency, either in direct destruction or by pollution of 

 the waters, is very great, and can only be compared to that of 

 some kinds of insects. Some years ago Guillemard described 



