10 INTRODUCTORY LECTURE. 



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 Istiophoriis 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 F<?rme5 mo//i^5ca, and the snail in a different class, viz. Vermes 

 testacea^ in the Systema Naturce; whilst in their whole anatomy 

 these two mollusks most closely resemble each other, the rudimental 

 state of the shell being the main difference in the Limax as com- 

 pared with the Helix. Similar instances of the violation of natural 

 affinities might be multiplied, and are, indeed, inevitable in an artifi- 

 cial 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 Linnoeus, and a necessary transitional step to 

 improved views on this subject, I should not have been justified in 

 occupying so much of the time of my auditors in this theatre of 

 anatomy and physiology, by the details of such artificial helps to the 

 recoo^nition of the outward characters of the members of the animal 

 kingdom. But the principle on which animals are now grouped to- 

 gether are of a different and much higher kind : they are the fruits 

 of the best results of the researches of all the great comparative 

 anatomists since the time of Linneeus. The characters of the classes 

 of animals have been rendered by the immortal Cuvier*, the highest 

 expressions of the facts ascertained in the animal organisation. I 

 know not any thing more calculated to impress the stranger to 

 anatomical science with the immensity of the labour that has been 

 gone through, and with the vast number of careful and minute dis- 

 sections that have been made, than the propositions which now form 

 the definitions of the primary groups of the animal kingdom. 



The whole organisation of one species has been compared with that 

 of another, and this with a third, and so on, in order to ascertain in 

 what organ, or system of organs, the greatest number of animals 

 would be found to present the same condition : so that they might 

 not be arbitrarily but naturally associated together. In the terms of 

 logic, the characters common to all animals having been ascertained, 



* XII. 



