SPECIES. 19 



ratively helpless condition, and that its mother is provided 

 with special mammary glands for its support \ 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 examine 

 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 de- 

 scended, 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 in- 

 dividual beings, which reappears in the product of generation 

 with certain invariable characters, and is constantly repro- 

 duced by the generative act of similar individuals." 



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

 which are so much alike that we may reasonably believe them 

 to have descended from a common stock, constitute a species'' 1 



From the above definitions it will be at once evident that 

 there are two leading ideas in the minds of zoologists when 



* 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 be hereafter explained at greater 

 length. 



