| ↑ 1 blocks not present in 1861 1869 1872; present in 1859 1860 1866 |
| In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile; and if this be so, we surely ought not to expect to find
sterility
both appearing and disappearing under nearly the same conditions of life.
|
|
| → to more uniform conditions during long periods of time, 1869 |
| during long periods of time to more uniform conditions, 1872 |
|
| → and been slowly modified 1869 |
| OMIT 1872 |
|
|
a few such cases, immediately to be given, have been observed. ↑ |
|
| The real difficulty in our present subject is not, as it appears to me, why domestic varieties have not become mutually infertile when crossed, but why this has so generally occurred with natural
as soon as they have been
in a sufficient
degree to take rank as species. We are far from precisely knowing the cause; nor is this surprising, seeing how profoundly ignorant we are in regard to the normal and abnormal action of the reproductive system. But we can see that species, owing to their struggle for existence with numerous competitors,
have been exposed
→to more uniform conditions during long periods of time,
than have
domestic varieties; and this may well make a wide difference in the result. For we know how commonly wild animals and plants, when taken from their natural conditions and subjected to captivity, are rendered sterile; and the reproductive functions of organic beings which have always lived
→and been slowly modified
under natural conditions would probably in like manner be eminently sensitive to the influence of an unnatural cross. Domesticated productions, on the other hand, which, as shown by the mere fact of their domestication, were not originally highly sensitive to changes in their conditions of life, and which can now generally resist with undiminished fertility repeated changes of conditions, might be expected to produce varieties, which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner. |
|
| I have as yet spoken as if the varieties of the same species were
fertile when intercrossed. But
|
