428 
CLASSIFICATION. 
Chap. XIII. 
tlie tkree elements of land, air, and water,—we can per¬ 
haps understand how it is that a numerical parallelism 
has sometimes been observed between the sub-groups 
in distinct classes. A naturalist, struck by a parallelism 
of this nature in any one class, by arbitrarily raising 
or sinking the value of the groups in other classes 
(and all our experience shows that this valuation 
has hitherto been arbitrary), could easily extend the 
parallelism over a wide range; and thus the septenary, 
quinary, quaternary, and ternary classifications have 
probably arisen. 
As the modified descendants of dominant species, 
belonging to the larger genera, tend to inherit the 
advantages, which made the groups to which they belong 
large and their parents dominant, they are almost sure 
to spread widely, and to seize on more and more places in 
the economy of nature. The larger and more dominant 
groups thus tend to go on increasing in size; and they 
consequently supplant many smaller and feebler groups. 
Thus Ave can account for the fact that all organisms, 
recent and extinct, are included under a few great 
orders, under still fewer classes, and all in one great 
natural system. As showing how few the higher groups 
are in number, and how widely spread they are through¬ 
out the world, the fact is striking, that the discovery of 
Australia has not added a single insect belonging to a 
new class; and that in the vegetable kingdom, as I 
learn from Dr. Hooker, it has added only tAvo or three 
orders of small size. 
In the chapter on geological succession I attempted 
to show, on the principle of each group having generally 
diverged much in character during the long-continued 
process of modification, how it is that the more ancient 
forms of life often present characters in some slight 
degree intermediate between existing groups. A feAV 
