28 
science of Ichthyology embraces, and when describing 
the various parts, both internal and external, of the 
body of a fish, as regards shape, appearance, function 
etc., he establishes a terminology for application to other 
similar descriptions in the future. Having thus intro- 
ductorily pointed out what characteristic features in ani- 
mals, more especially fishes, ought to be studied by 
the ichthyologist, - - in doing which he finds occasion 
to remark that the blood-vessels, the lymphatic ducts 
and the nerves may be passed over, as being too spe- 
cial and as more properly belonging to the domain of 
Comparative Anatomy --he goes on to make clear for 
what purpose the knowledge in question should be ac- 
quired, that being to enable us to arrange and subdi- 
vide fishes systematically. 
Now it is not only in Ichthyology that certain gen- 
era show points of agreement with one another; the 
same holds good in all branches of natural history. 
Thus, to take an example, the claw r ed mammals all re- 
semble each other, and the same is true of those that 
are hoofed; if, however, a clawed genus be compared 
with a hoofed, they will be found to differ in most 
particulars. This shows that it is possible to arrange 
animals for scientific purposes in main divisions or 
classes, and likewise plants, &c. These classes may be 
either artificial, or hypothetical, or natural and true 
divisions, depending upon what ground of classification 
is chosen. It is one of ARTEDI'S great claims to be re- 
membered with honour, that he was the first to advo- 
cate natural classes, and to show how utterly unscien- 
tific it was to choose as the ground of classification, as 
had been done in ichthyological treatises before his time, 
either the habitat 1 of the animals, their size, etc., or 
the number of their external organs (e. g. fins). The 
instance of fins w y ill probably have been adduced as a 
criticism of WILLUGHBY'S work, for he employed the 
1 However astonishing it may, seem, there really existed at one 
time a quasi-scientific classification of fishes into: Sea-fishes, River- 
fishes, Lake-fishes and Marsh-fishes (by RONDELETIUS). 
