CHAPTER SECOND. 



LEADING GROUPS OF THE EXISTING SYSTEMS 

 OF ANIMALS. 



SECTION I. 



GREAT TYPES OR BRANCHES OF THE ANIMAL KINGDOM. 



THE use of the terms types, classes, orders, families, genera 

 and species in the systems of Zoology and Botany is so 

 universal, that it would be natural to suppose that their 

 meaning and extent are well determined and generally 

 understood; but this is so far from being the case, that it 

 may, on the contrary, be said there is no subject in Natu- 

 ral History respecting which there exists more uncer- 

 tainty, and a greater want of precision. Indeed, I have 

 failed to find anywhere a definition of the character of 

 most of the more comprehensive of these divisions, while 

 the current views respecting genera and species are very 

 conflicting. Under these circumstances, it has appeared 

 to me particularly desirable to inquire into the foundation 

 of these distinctions, and to ascertain, if possible, how far 

 they have a real existence. And, while I hope the results 

 of this inquiry may be welcome and satisfactory, I am 

 free to confess that it has cost me years of labour to arrive 

 at a clear conception of their true character. 



It is a fact so universal, in every sphere of intellec- 

 tual activity, that practice anticipates theory, that no 



