same subgroup have different habits. In the second and fourth
chapters, on Variation and on Natural Selection, I have attempted
to show that within each country it is the widely ranging, the much
diffused and common, that is the dominant species, belonging to
the larger genera in each class, which vary most. The varieties, or
incipient species, thus produced, ultimately become converted into new
and distinct species; and these, on the principle of inheritance, tend
to produce other new and dominant species. Consequently the groups which
are now large, and which generally include many dominant species, tend
to go on increasing 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, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms, which, in any small area, come
into the closest competition, and by certain facts in naturalisation.

I attempted also to show that there is a steady tendency in the forms
which are increasing in number and diverging in character, to supplant
and exterminate the preceding, less divergent and less improved 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 subordinate to groups. In
the diagram each letter on the uppermost line may represent a genus
including several species; and the whole of the genera along this upper
line form together one class, for all are descended from one ancient
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 subfamily, distinct from that containing 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 in common,
though less than when grouped in subfamilies; and they form a family
distinct from that containing the three genera still further to the
right hand, which diverged at an earlier period. And all these genera,
descended from (A), form an order distinct from the genera descended
from (I). So that we here have many species descended from a single
progenitor grouped into genera