CLASSIFICATION. 2/ 



port ; this expressing a fundamental distinction from all fishes, 

 and being associated with other equally essential correlations 

 of structure. 



The entire animal kingdom is primarily divided into some 

 half-a-dozen great plans of structure, the divisions thus formed 

 being called " sub-kingdoms/' The sub-kingdoms are, in turn, 

 broken up into classes, classes into orders, orders into families, 

 families into genera, and genera into species. We shall ex- 

 amine these successively, commencing with the consideration 

 of a species, since this is the zoological unit of which the 

 larger divisions are made up. 



Species. No term is more difficult to define than " species," 

 and on no point are zoologists more divided than as to what 

 should be understood by this word. Naturalists, in fact, are 

 not yet agreed as to whether the term species expresses a real 

 and permanent distinction, or whether it is to be regarded 

 merely as a convenient, but not immutable, abstraction, the 

 employment of which is necessitated by the requirements of 

 classification. 



By Buffon " species " is defined as " a constant succession 

 of individuals * similar to and capable of reproducing each 

 other." 



De Candolle defines species as an assemblage of all those 

 individuals which resemble each other more than they do 

 others ; and are able to reproduce their like, doing so by the 

 generative process, and in such a manner that they may be 

 supposed by analogy to have all descended from a single being 

 or a single pair. 



M. de Quatrefages defines species as "an assemblage of 

 individuals, more or less resembling one another, which are 

 descended, or may be regarded as being descended, from 

 a single primitive pair by an uninterrupted succession of 

 families." 



Miiller defines species as "a living form, represented by 

 individual beings, which reappears in the product of generation 

 with certain invariable characters, and is constantly reproduced 

 by the generative act of similar individuals." 



According to Pritchard, a species is constituted by " separate 

 origin, and distinctness of race, evinced by a constant trans- 

 mission of some characteristic peculiarity of organisation." 



According to Woodward, " all the specimens, or individuals, 



* In using the term "individual," it must be borne in mind that the 

 "zoological individual" is meant; that is to say, the total result of the 

 development of a single ovum, as will hereafter be explained at greater 

 length. 



