18 Principles of Plant Culture. 
the white oak, the red oak and the bur oak; the raspberry 
and the blackberry; and the apple, pear and quince are 
formed into groups, each of which is called a genus, (ge’-nus), 
plural, genera, (gen’-e-ra). Then che genera that resemble 
each other, as the one containing the apple, pear and quince, 
and the one containing the plum, cherry and peach, are 
formed into other groups called families.* Thus families 
are made up of genera, and genera are made up of species. 
There may he, also, different varieties in the same species, as 
the different varieties of apple, pea or strawberry. 
An extensive retail bookstore furnishes an object lesson 
in classification, though we must remember tbat, in natural 
history, it is usually the names and descriptions of plants 
and animals that are classified, and not the plants and ani- 
mals themselves. In the bookstore, we will observe that 
the books are not placed upon the shelves without order, but 
that they are arranged in groups. Different copies of the 
same work are placed together. Different works on the same 
subject, as Gray’s botany, Wood's botany, Bessey’s botany 
are also placed in a larger group. Then all the scientific 
books are formed into a still larger group, as are the books 
of fiction, the books of poetry, the music books, ete. Com- 
paring this arrangement with that employed in natural his- 
tory, each separate work, as Gray’s Manual of Botany, 
Thomas’ Fruit Culturist, Bunyan’s Pilgrim’s Progress, etc., 
would correspond to a species, and the different copies of 
the same work would correspond to individuals. The books 
treating of the same general subject, as the different works 
on geology, botany or arithmetic would correspond to 
genera, and the different classes of books, as scientific 
books, books of fiction, etc., would correspond to families. 
* Groups of related families are often further untted into orders. 
