CATEGORIES OF CLASSIFICATION. 31 
These divisions are, first, the most comprehen- 
sive groups, the primary divisions, called branches 
by some, types by others, and divided by some 
naturalists into so-called sub-types, meaning only 
a more limited circumscription of the same kind 
of group ; next we have classes, and these also 
have been divided into sub-classés; then orders 
and sub-orders; families and sub-families or 
tribes; then genera, species, and varieties. With 
reference to the question whether these groups 
really exist in Nature, or are merely the expres- 
sion of individual theories and opinions, it is 
worth while to study the works of the early natu- 
ralists, in order to trace the natural process by 
which scientific classification has been reached ; 
for in this, as in other departments of learning, 
practice has always preceded theory. We do the 
thing before we understand why we do it: speech 
precedes grammar, reason precedes logic ; and so 
a division of animals into groups, upon an in- 
stinctive perception of their differences, has pre- 
ceded all our scientific creeds and doctrines. Let 
us, therefore, proceed to examine the meaning 
of these names as adopted by naturalists. 
When Cuvier proposed his four primary di- 
visions of the animal kingdom, he added his 
argument for their adoption, — because, he said, 
they are constructed on four different plans. 
All the progress in our science since his time 
