THE MEANING OF THE WORD SPECIES. 291 



The word " species " in Biology has two significations ; 

 the one based upon morphological, the other upon 

 physiological considerations. 



A species, in the strictly morphological sense, is simply 

 an assemblage of individuals which agree with one another, 

 and differ from the rest of the living world in the sum 

 of their morphological characters ; that is to say, in 

 the structure and in the development of both sexes. 

 If the sum of these characters in one group is repre- 

 sented by A, and that in another by A + n ; the two 

 are morphological species, whether n represents an 

 important or an unimportant difference. 



The great majority of species described in works on 

 Systematic Zoology are merely morphological species. 

 That is to saj r , one or more specimens of a kind of animal 

 having been obtained, these specimens have been found 

 to differ from any previously known by the character or 

 characters n; and this difference constitutes the defi- 

 nition of the new species, and is all we really know 

 about its distinctness. 



But, in practice, the formation of specific groups is 

 more or less qualified by considerations based upon what 

 is known respecting variation. It is a matter of obser- 

 vation that progeny are never exactly like their parents, 

 but present small and inconstant differences from them. 

 Hence, when specific identity is predicated of a group of 

 individuals, the meaning conveyed is not that they are 

 all exactly alike, but only that their differences are so 



