ON CLASSIFICATION. 13 



A Species is, as it were, the unit of which all 

 groups are formed. All the individuals of a species 

 resemble each other both in structure and habits ; 

 they breed freely together, and their progeny resemble 

 themselves. 



The only difference between individuals of a species 

 is that due to sex ; and among fishes there is seldom 

 any perceptible outward distinction between the male 

 and the female. A species may be either considered 

 as a separate creation, or, in the view which is now 

 more generally held, as a stereotyped variety of some 

 older form. The latter view binds the entire orga- 

 nised creation into an harmonious system. Giving 

 reins to our imagination, we discern, in the mind's 

 eye, the various orders as vast branches of some 

 primaeval form of fathomless antiquity, each in pro- 

 cess of time, as they increased and multiplied and 

 scattered themselves in various climates, giving rise 

 to divergent groups varying one from another, which 

 we now rank as families ; these again splitting up into 

 genera, and each genus dividing into few or many 

 separate forms which we call species. 



There is little difficulty, with fishes at any rate, in 

 discriminating one species from another (if we only 

 have a sufficient number of specimens of each kind to 

 compare together). 



It is certainly quite conceivable that two species 

 should exist, bearing the most perfect resemblance 

 one to the other, in the minutest particulars, and yet 

 quite distinct. It is possible to imagine that two 



