THE MEANING OF THE WORD SPECIES. 291 



The word " species " in Biology has two significations ; 

 the one hased upon morphological, the other upon 

 physiological considerations. 



A species, in the strictly morphological sense, is simply 

 an asserahlage of individuals which agree with one another, 

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

 of then- 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 + w ; 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^ one or more specimens of a kind of animal 

 having been obtained, these specimens have been found 

 to differ from any previously known b}^ 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, m 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, tiie meaning conveyed is not that they are 

 all exactly alike, but only that their differences are so 



