412 
CLASSIFICATION. 
Chap. XIIL 
species. Consequently the groups which are now large, 
and which generally include many dominant species, 
tend to go on increasing indefinitely in size. I further 
attempted to show that from the varying descendants of 
each species trying to occupy as many and as different 
places as possible in the economy of nature, there is a 
constant tendency in their characters to diverge. This 
conclusion was supported by looking at the great diversity 
of the forms of life which, in any small area, come into 
the closest competition, and by looking to certain facts 
in naturalisation. 
I attempted also to show that there is a constant 
tendency in the forms which are increasing in number 
and diverging in character, to supplant and exterminate 
the less divergent, the less improved, and preceding 
forms. I request the reader to turn to the diagram 
illustrating the action, as formerly explained, of these 
several principles; and he will see that the inevitable 
result is that the modified descendants proceeding from 
one progenitor become broken up into groups subordi¬ 
nate to groups. In the diagram each letter on the 
' uppermost line may represent a genus including several 
species; and all the genera on this line form together 
one class, for all have descended from one ancient 
but unseen parent, and, consequently, have inherited 
something in common. But the three genera on the 
left hand have, on this same principle, much in common, 
and form a sub-family, distinct from that including the 
next two genera on the right hand, which diverged from 
a common parent at the fifth stage of descent. These 
five genera have also much, though less, in common; 
and they form a family distinct from that including 
the three genera still further to the right hand, which 
diverged at a still earlier period. And all these genera, 
descended from (A), form an order distinct from the 
