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science of Ichthyology embraces, and when describing 

 the various parts, both internal and external, of the 

 body of a fish, as regards shape, appearance, function 

 etc., he establishes a terminology for application to other 

 similar descriptions in the future. Having thus intro- 

 ductorily pointed out what characteristic features in ani- 

 mals, more especially fishes, ought to be studied by 

 the ichthyologist, — in doing which he finds occasion 

 to remark that the blood-vessels, the lymphatic ducts 

 and the nerves may be passed over, as being too spe- 

 cial and as more properly belonging to the domain of 

 Comparative Anatomy — he goes on to make clear for 

 what purpose the knowledge in question should be ac- 

 quired, that being to enable us to arrange and subdi- 

 vide fishes systematically. 



Now it is not only in Ichthyology that certain gen- 

 era show points of agreement with one another; the 

 same holds good in all branches of natural history. 

 Thus, to take an example, the clawed mammals all re- 

 semble each other, and the same is true of those that 

 are hoofed; if, however, a clawed genus be compared 

 with a hoofed, they will be found to differ in most 

 particulars. This shows that it is possible to arrange 

 animals for scientific purposes in main divisions or 

 classes, and likewise plants, &c. These classes may be 

 either artificial, or hypothetical, or natural and true 

 divisions, depending upon what ground of classification 

 is chosen. It is one of Artedi's great claims to be re- 

 membered with honour, that he was the first to advo- 

 cate natural classes, and to show how utterly unscien- 

 tific it was to choose as the ground of classification, as 

 had been done in ichthyological treatises before his time, 

 either the habitat 1 of the animals, their size, etc., or 

 the number of their external organs (e. g. fins). The 

 instance of fins will probably have been adduced as a 

 criticism of Willughby's work, for he employed the 



1 However astonishing it may seem, there really existed at one 

 time a quasi-scientific classification of fishes into: Sea-fishes, River- 

 fishes, Lake-fishes and Marsh-fishes (b}^ Rondeletius). 



