affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins. I think no better evidence of this can be required than that the two most experienced observers who have ever lived,
namely, namely, 1859 1860 1861 1866 1869 | namely 1872 |
Kölreuter and Gärtner,
should have should have 1859 1860 1861 1866 1869 | should have 1872 |
arrived at diametrically opposite conclusions in regard to
the the 1859 1860 1861 1866 1869 |
some of the 1872 |
very same
species. species. 1859 1860 1861 1866 1869 | forms. 1872 |
It is also most instructive to
compare—but compare—but 1866 1869 | compare— 1859 1860 1861 1872 |
....... 1866 1869 | but 1859 1860 1861 1872 |
I have not space here to enter on
details—the details—the 1866 1869 | details— 1859 1860 1861 1872 |
....... 1866 1869 | the 1859 1860 1861 1872 |
evidence advanced by our best botanists on the question whether certain doubtful forms should be ranked as species or varieties, with the evidence from fertility adduced by different hybridisers, or by the same
author, author, 1859 1860 1861 1866 1869 | observer 1872 |
from experiments made during different years. It can thus be shown that neither sterility nor fertility affords any
clear clear 1859 1860 1861 1866 1869 | certain 1872 |
distinction between species and
varieties; varieties; 1859 1860 1861 1866 1869 | varieties. 1872 |
but that the but that the 1859 1860 1861 1866 1869 |
The 1872 |
evidence from this source graduates away, and is doubtful in the same degree as is the evidence derived from other constitutional and structural differences. |
|
In regard to the sterility of hybrids in successive generations; though Gärtner was enabled to rear some hybrids, carefully guarding them from a cross with either pure parent, for six or seven, and in one case for ten generations, yet he asserts positively that their fertility never
increased, increased, 1859 1860 1861 1866 1869 | increases, 1872 |
but generally
....... 1866 1869 1872 | greatly 1859 1860 1861 |
decreased greatly and suddenly. decreased greatly and suddenly. 1866 1869 |
decreased. 1859 1860 1861 |
decreases greatly and suddenly. 1872 |
With respect to this decrease, it may first be noticed that, when any deviation in structure or constitution is common to both parents, this is often transmitted in an augmented degree to the offspring; and both sexual elements in hybrid plants are already affected in some degree. ↑1 blocks not present in 1866 1869; present in 1859 1860 1861 1872 | I do not doubt that
this is usually the case, and that the fertility often suddenly decreases in the first few generations.
|
But But 1866 1869 1872 | Nevertheless 1859 1860 1861 |
I believe
....... 1866 1869 | that 1859 1860 1861 1872 |
in nearly all these cases, that the in nearly all these cases, that the 1866 1869 |
in all these experiments the 1859 1860 1861 |
their 1872 |
fertility has been diminished
by by 1859 1860 1861 1866 1869 |
in nearly all these cases by 1872 |
an independent cause, namely,
by too by too 1866 1869 1872 | from 1859 1860 1861 |
close interbreeding. I have
collected collected 1859 1860 1861 1866 1869 | made 1872 |
so
large a body of large a body of 1859 1860 1861 1866 1869 |
many experiments and collected so many 1872 |
facts,
|