of the same capsule and exposed to
....... 1861 1866 1869 1872 | exactly 1859 1860 |
the same conditions. |
|
By the term systematic affinity is meant, the
resemblance resemblance 1859 1860 1861 1866 1869 | general resemblance 1872 |
between species in structure and
in in 1859 1860 1861 1866 1869 | in 1872 |
constitution, constitution, 1859 1860 1861 1866 1869 | constitution. 1872 |
more more 1859 1860 1861 1866 1869 | more 1872 |
especially especially 1859 1860 1861 1866 1869 | especially 1872 |
in in 1859 1860 1861 1866 1869 | in 1872 |
the the 1859 1860 1861 1866 1869 | the 1872 |
structure structure 1859 1860 1861 1866 1869 | structure 1872 |
of of 1859 1860 1861 1866 1869 | of 1872 |
parts parts 1859 1860 1861 1866 1869 | parts 1872 |
which which 1859 1860 1861 1866 1869 | which 1872 |
are are 1859 1860 1861 1866 1869 | are 1872 |
of of 1859 1860 1861 1866 1869 | of 1872 |
high high 1859 1860 1861 1866 1869 | high 1872 |
physiological physiological 1859 1860 1861 1866 1869 | physiological 1872 |
importance importance 1859 1860 1861 1866 1869 | importance 1872 |
and and 1859 1860 1861 1866 1869 | and 1872 |
which which 1859 1860 1861 1866 1869 | which 1872 |
differ differ 1859 1860 1861 1866 1869 | differ 1872 |
little little 1859 1860 1861 1866 1869 | little 1872 |
in in 1859 1860 1861 1866 1869 | in 1872 |
the the 1859 1860 1861 1866 1869 | the 1872 |
allied allied 1859 1860 1861 1866 1869 | allied 1872 |
species. species. 1859 1860 1861 1866 1869 | species. 1872 |
Now the fertility of first
crosses crosses 1859 1860 1861 1866 1869 | crosses, 1872 |
between species, between species, 1859 1860 1861 1866 1869 | between species, 1872 |
and of the hybrids produced from them, is largely governed by their systematic affinity. This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. But the correspondence between systematic affinity and the facility of crossing is by no means strict. A multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the utmost facility. In the same family there may be a genus, as Dianthus, in which very many species can most readily be crossed; and another genus, as Silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid. Even within the limits of the same genus, we meet with this same difference; for instance, the many species of Nicotiana have been more largely crossed than the species of almost any other genus; but Gärtner found that N. acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised
by, by, 1859 1860 1861 1866 1869 | by 1872 |
no less than eight other species of Nicotiana.
....... 1869 1872 | Very 1859 1860 1861 1866 |
Many Many 1869 1872 | many 1859 1860 1861 1866 |
analogous facts could be given. |
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No one has been able to point out what
kind, kind, 1859 1860 1861 1866 1869 | kind 1872 |
or what
amount, amount, 1859 1860 1861 1866 1869 | amount 1872 |
of
difference difference 1859 1860 1861 1866 1869 | difference, 1872 |
in any recognisable
character character 1859 1860 1861 1866 1869 | character, 1872 |
is sufficient to prevent two species crossing. It can be shown that plants most widely different in habit and
|