→
p
14
1860 |
p
14
,
1859 1861 1866 1869 1872 |
|
→
f
14
1860 |
f
14
,
1859 1861 1866 1869 1872 |
|
→
m
14
1860 |
m
14
,
1859 1861 1866 1869 1872 |
|
→ and as these latter two genera, both from continued divergence of character and from inheritance from a different parent, will differ 1859 1860 1861 1866 |
differing 1869 1872 |
|
→ three genera descended from (A), the two little 1859 1860 1861 1866 |
descendants of (A). These two 1869 1872 |
|
→ one species of a 1859 1860 1861 1866 |
some 1869 1872 |
|
the extinct species lived at
epochs when the branching lines of descent had diverged less. |
|
I see no reason to limit the process of modification, as now explained, to the formation of genera alone. If, in
diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be
great, the forms marked
a
14
to
→
p
14
marked
b
14
and
→
f
14
those marked
o
14
to
→
m
14
form three very distinct genera. We shall also have two very distinct genera descended from
→and as these latter two genera, both from continued divergence of character and from inheritance from a different parent, will differ
widely from the
→three genera descended from (A), the two little
groups of genera will
two distinct families, or
orders, according to the amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders,
descended from two species of the original
and these
are supposed to
descended from
→one species of a
still more ancient and unknown
|
|
We have seen that in each country it is the species
the larger genera which oftenest present varieties or incipient species. This, indeed, might have been expected;
as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group shows that its species have inherited from a common ancestor some advantage in common. Hence, the struggle for the production of new and modified
will mainly lie between the larger
which are all trying to increase in number. One large group will slowly conquer another large group, reduce its numbers, and thus lessen its chance of further variation and improvement. Within the same large
|