10 INTRODUCTORY LECTURE. 



venieiice of their appreciation, thus tend to the association of very 

 differently organised species ; and, on the other hand, they are equally 

 liable to separate into very remote groups of an artificial system, 

 two animals which may have very similar anatomical structures. 

 Of this we have several examples in the Linnaean subdivisions of the 

 class of fishes, the orders of which are characterised by the easily 

 recognisable position of the fins. Linnaeus's attention was par- 

 ticularly directed to the very variable position of the ventral pair of 

 fins, which are the analogues of the hinder limbs in land animals. 

 In some fishes, as the pike and many other fresh-water species, the 

 ventral fins are at some distance behind the pectoral fins, or in their 

 usual place — these formed the order Abdomincdes : in others, as the 

 perch, the ventral fins are attached beneath the thorax — these 

 constituted the Thoracic order : in others, as the cod, you find the 

 ventral fins in advance of the pectorals, or under the throat — such 

 species formed the Pisces jugulates of Linnaeus : lastly, those 

 species in which the ventral fins are altogether wanting, as the eel, 

 formed the Apodal order. 



Such a system has the advantage of enabling the collector to refer 

 with great facility any fish to its artificial order ; but you can scarcely 

 express any general proposition in comparative anatomy in reference 

 to such groups. There are two sword-fishes, for example, having the 

 same anatomical structure, and not easily distinguishable externally 

 save by the height of the dorsal and the difference in the position of 

 the ventral fins : but in the Systema Naturce of the Swedish Na- 

 turalist, the Xiphias is placed in one order, and the Istiophorus in 

 another ; the variable and little influential fins prevailing over all the 

 rest of the organisation in the artificial ichthyology of Linnaeus. 



Amongst the lower animals, we find the slug, placed in one class, 

 viz. the Vermes mollusca^ and the snail in a different class, viz. Vermes 

 testacea, in the Systema NaturcE ; whilstin their wholeanatomy these two 

 mollusca most closely resemble each other, the rudimental state of the 

 shell being the main difference in the Limax or slug. Similar instances 

 of the violation of natural affinities might be multiplied, and are, indeed, 

 inevitable in an artificial system. 



I confess that if the classifications of zoology of the present day 

 continued to be of the same character as that to which I have just 

 referred, which however, let it be remembered, was the best that could 

 be made in the time of Linnaeus, and a necessary transitional step to 

 improved view's on this subject, I should not have been justified in 

 occupying the time of the auditors in this theatre of anatomy and phy- 

 siology, by the details of such artificial helps to the recognition 

 of the outward characters of the members of the animal kingdom. 



