CLASSIFICATION OK ANIMALS. 9 



turc. Sonic knowledge of natural history and of the principles of 

 classification, therefore, is essential to the comprehension of the con- 

 nection between structure and habits, on which the utility of Anatomy 

 in the advancement of Physiology mainly depends. 



The classification of animals is not now what it was in the time of 

 Linnoeus. I do not mean merely to say that animals are differently 

 arranged, but the objects and principles of that arrangement are very 

 different. 



Linnaeus in his Systema Naturae wished to give, as it were, a 

 Dictionary of the Animal Kingdom, by reference to which you might 

 as readily ascertain the place of the animal in his system as that of a 

 word in a lexicon by merely knowing its first and second letters. 

 To this end, Linnaius selected a few of the most obvious characters 

 for the establishment of his groups. 



Taking, for example, a certain number of incisor teeth and the 

 pectoral position of the mammae, as the characters of his first order of 

 animals, he thereby associated man with the monkeys and the bats. 

 But, independently of the psychical endowments which place the 

 human species far above the lower creation, it may readily be con- 

 ceived that great diffei'ences of organisation must exist in animals 

 which enjoy the erect position on two feet, in those which climb by 

 having four hands, and in those which fly by having their anterior 

 members in the form of wings. 



External and arbitrary characters, selected merely for the con- 

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

 differently organised species, and as often separate into very re- 

 mote groups of an artificial system two animals which may have 

 very similar anatomical structures. Of this we have several ex- 

 amples in the Linnaean subdivisions of the class of fishes, the orders 

 of which are characterised by the easily recognisable position of the 

 fins. Linneeus's attention was particularly directed to the very vari- 

 able position of the ventral pair of fins, which are the homologues 

 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 Abdommales : in others, as the perch, the ventral fins arc 

 attached beneath the tliorax — these constitute 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 jugu- 

 lares of Linnaius : 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 



