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 
