ON CLASSIFICATION. 15 



Thus it sometimes happens that one single species 

 is so far removed from its nearest allies that natural- 

 ists have to place it alone in a distinct family from all 

 other species. Consequently there seem to be many 

 blanks left in the series, the intermediate forms may 

 be unknown to us, or else the existing form may be 

 the only one that survives out of a large group, the 

 remainder of which have become extinct. 



We have seen that when two species that are really 

 distinct are placed before us, and we are able to com- 

 pare them critically one with the other, there is 

 seldom any difficulty in discriminating between them. 

 There is sometimes difficulty in knowing what group 

 a fish belongs to ; this is generally in consequence of 

 the imperfection of our classification, and becomes 

 less in proportion as our genera form natural groups. 



There is, however, often a difficulty with fishes that 

 in reality are of the same species, either they may 

 have been described by different authors under dis- 

 tinct names, and each author may have laid stress on 

 different points : in this case by comparing the de- 

 scriptions alone, it is impossible to be certain if the 

 fishes are the same or different : or else the species 

 may be variable in itself, which often happens when 

 it has a large geographical range, specimens from one 

 locality differing from those of another, or the same 

 individual appearing different at different times, 

 according to the food it is living on, the kind of water 

 it is inhabiting, or the season of the year. 



Herein lies the great value of a large and carefully 



