| → species. 1859 1860 1866 |
| species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result. 1861 |
|
| ↑ 5 blocks not present in 1859 1860 1861 1866; present in 1869 1872 |
| 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 varieties
as soon as they have been modified
in a sufficient and permanent
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, must
have been exposed to more uniform conditions during long periods of time,
than have been
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.
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|
| → is 1861 1866 1869 1872 |
| seems to me 1859 1860 |
|
|
that a long course of domestication tends to eliminate sterility; 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. Lastly, and this seems to me
the most important consideration, new races of animals and plants are produced under domestication
man's methodical and unconscious power of selection, for his own use and pleasure: he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system. Domestic productions are less closely adapted to climate and to the other physical conditions of the countries which they inhabit than are those in a state of nature, for they can generally be removed to other and differently constituted countries with
impunity.
supplies his several varieties with the same food;
them in nearly the same
and does not wish to alter their general habits of life. Nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature's own good; and thus she may, either directly, or more probably indirectly, through correlation, modify the reproductive
the several descendants from any one
→species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result. ↑ |
|
| I have as yet spoken as if the varieties of the same species were
fertile when intercrossed. But it
→is
impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which I will briefly abstract. The evidence is at least as good as that from which we believe in the sterility of a multitude of species. The evidence is, also, derived from hostile witnesses, who in all other
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