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.
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