st chance of filling new and widely different
places in the polity of nature: hence in the diagram I have chosen the
extreme species (A), and the nearly extreme species (I), as those which
have largely varied, and have given rise to new varieties and species.
The other nine species (marked by capital letters) of our original
genus, may for a long period continue transmitting unaltered
descendants; and this is shown in the diagram by the dotted lines not
prolonged far upwards from want of space.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency in
the improved descendants of any one species to supplant and exterminate
in each stage of descent their predecessors and their original parent.
For it should be remembered that the competition will generally be most
severe between those forms which are most nearly related to each other
in habits, constitution, and structure. Hence all the intermediate forms
between the earlier and later states, that is between the less and more
improved state of a species, as well as the original parent-species
itself, will generally tend to become extinct. So it probably will be
with many whole collateral lines of descent, which will be conquered by
later and improved lines of descent. If, however, the modified offspring
of a species get into some distinct country, or become quickly adapted
to some quite new station, in which child and parent do not come into
competition, both may continue to exist.

If then our diagram be assumed to represent a considerable amount of
modification, species (A) and all the earlier varieties will have become
extinct, having been replaced by eight new species (a14 to m14); and (I)
will have been replaced by six (n14 to z14) new species.

But we may go further than this. The original species of our genus were
supposed to resemble each other in unequal degrees, as is so generally
the case in nature; species (A) being more nearly related to B, C, and
D, than to the other species; and species (I) more to G, H, K, L, than
to the others. These two species (A) and (I), were also supposed to be
very common and widely diffused species, so that they must originally
have had some a