6 A HISTORY OF FISHES 



separated from the Bony Fishes for a very long period of the 

 earth's history [cf. p. 350). In speaking of fishes, therefore, it 

 must be remembered that we are referring to three very distinct 

 vertebrate classes, here grouped together merely for the sake of 

 convenience. 



Certainly in number of individuals, and probably also in 

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

 mammals, birds, reptiles, or amphibians. Recollecting that 

 three-quarters of the earth's surface is covered by the seas, and 

 that many of the fresh waters of the land teem with fish life, 

 this superiority of numbers is easier to understand. The sur- 

 faces of the great oceans, their middle layers, the abyssal depths 

 and shore regions; the estuaries, mighty rivers, swiftly flowing 

 brooks, turbulent mountain torrents and placid lakes and 

 ponds; each of these possesses its peculiar forms of fish life, 

 variously modified according to circumstances. There are 

 probably more than 20,000 different species of fish in existence 

 to-day, and 100 or more new forms seem to be discovered 

 every year. Aristotle seems to have been familiar with only 

 about 115 species, all of them found in the iEgean Sea. Pliny 

 {circa a.d. 200), whose list included as many as 176 species, 

 triumphantly exclaims: "In the sea and in the ocean, vast as 

 it is, there exists, by Hercules! nothing that is unknown to us, 

 and a truly marvellous feat it is that we are best acquainted 

 with those things which Nature has concealed in the deep." 

 Sancta simplicitas! Regarding the number of individuals of any 

 particular species, it is wellnigh impossible to give an adequate 

 idea of their abundance. It has been estimated that nearly 

 400,000,000 Cod and more than 3,000,000,000 Herring are 

 caught each year in the Atlantic and adjacent seas alone, and 

 these numbers must represent but a minute proportion of the 

 individuals in existence at a given time. 



The particular branch of zoology which treats of the 

 structure of fishes, both external and internal, their mode of 

 life, their distribution in space and time, etc., is known as 

 "ichthyology," a word derived from the Greek ichthys, a fish, 

 and logos^ a discourse. The scope of ichthyology is enormous, 

 and it is almost impossible to deal with its many branches 

 within the compass of a single volume. The anatomist, in- 

 vestigating and comparing the internal structure of the various 

 kinds of fishes ; the embryologist, concerned with the develop- 

 ment of the individual from the Ggg to the adult; the evolu- 

 tionist, studying the past history of the group; the systematist 



