| → we plainly see that 1859 1860 1861 1866 |
| OMIT 1869 1872 |
|
| → that is between members of different strains or sub-breeds, 1859 1860 1861 1866 |
| which differ to a certain extent, 1869 1872 |
|
| → offspring. 1859 1860 1861 1866 |
| offspring; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, almost always induces weakness and sterility. 1869 |
| offspring; 1872 |
|
| → I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; 1859 1860 1861 1866 |
| OMIT 1872 |
|
| → always induces weakness and sterility in the progeny. 1859 1860 1861 1866 |
| almost always leads to decreased size, weakness, or sterility. 1872 |
|
| → varied and become 1859 1860 1861 1866 1869 |
| been subjected to 1872 |
|
| → give 1859 1860 1861 1866 1869 |
| conditions, or which have slightly varied, give 1872 |
|
| → that greater changes, or changes of a particular nature, often render 1859 1860 1861 1866 1869 |
| OMIT 1872 |
|
| → in some degree 1859 1860 1861 1866 1869 |
| long habituated to certain uniform conditions under a state of nature, when subjected, as under confinement, to a considerable change in their conditions, very frequently are rendered more or less 1872 |
|
| → that greater crosses, that is 1859 1860 1861 1866 1869 |
| we know that a 1872 |
|
| → males and females which 1859 1860 1861 1866 1869 |
| two forms, that 1872 |
|
| → cannot persuade myself 1859 1860 1861 1866 1869 |
| am fully persuaded 1872 |
|
| → parallelism is 1859 1860 1861 1866 1869 |
| double parallelism is by no means 1872 |
|
| ↑ 3 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| He who is able to explain why the elephant and a multitude of other animals are incapable of breeding when kept under only partial confinement in their native country, will be able to explain the primary cause of hybrids being so generally sterile.
He will at the same time be able to explain how it is that the races of some of our domesticated animals, which have often been subjected to new and not uniform conditions, are quite fertile together, although they are descended from distinct species, which would probably have been sterile if aboriginally crossed.
The above two parallel series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life; this principle, according to Mr. Herbert Spencer, being that life depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces gain in power.
|
|
| → life. 1859 1860 1861 |
| life; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this ten- dency is slightly disturbed by any change, the vital forces apparently gain in power. 1866 |
| life; this principle apparently being that life, as Mr. Herbert Spencer has remarked, depends on, or consists in, the incessant action and reaction of various forces, which, as throughout nature, are always tending towards an equilibrium; and when this tendency is slightly disturbed by any change, the vital forces apparently gain in power. 1869 |
|
| ←Subtitle not present 1859 1860 1861 Reciprocal
Dimorphism
and
Trimorphism
.
1866 1869 |
| ↑ 31 blocks not present in 1859 1860 1861; present in 1866 1869 1872 |
| This subject may be here briefly discussed, and will be found to throw considerable
light on hybridism.
Several plants belonging to distinct orders present two forms, existing together
in about equal numbers,
which
differ in no respect except in their reproductive organs; one form having a long pistil with short stamens, the other a short pistil with long stamens; both with
differently sized pollen-grains.
With trimorphic plants there are three forms likewise differing in the lengths of their pistils and stamens, in the size and colour of the pollen-grains, and in some other respects; and as in each of the three forms there are two sets of stamens, there are
altogether six sets of stamens and three kinds of pistils.
These organs are so proportioned in length to each other, that,
in any
two of the forms,
half the stamens in each
stand on a level with the stigma of the third form.
Now I have shown, and the result has been confirmed by other observers, that, in order to obtain full fertility with these plants, it is necessary that the stigma of the one form should be fertilised by pollen taken form
the stamens of corresponding height in the
other
form.
So that with dimorphic species two unions, which may be called legitimate, are fully fertile,
and two, which may be called illegitimate, are more or less infertile.
With trimorphic species six unions are legitimate or fully fertile, and twelve are illegitimate or more or less infertile.
The infertility which may be observed in various dimorphic and trimorphic plants, when they are illegitimately fertilised, that is by pollen taken from stamens not corresponding in height with the pistil, differs much in degree, up to absolute and utter sterility; just in the same manner as occurs in crossing distinct species.
As the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or less favourable, so I have found it with illegitimate unions.
It is well known that if pollen of a distinct species be placed on the stigma of a flower, and its own pollen be afterwards, even after a considerable interval of time, placed on the same stigma, its action is so strongly prepotent that it generally annihilates the effect of the foreign pollen; so it is with the pollen of the several forms of the same species, for legitimate pollen is strongly prepotent over illegitimate pollen, when both are placed on the same stigma.
I ascertained this by fertilising several flowers, first illegitimately, and twenty-four hours afterwards legitimately, with pollen taken from a peculiarly coloured variety, and all the seedlings were similarly coloured; this shows that the legitimate pollen, though applied twenty-four hours subsequently, had wholly destroyed or prevented the action of the previously applied illegitimate pollen.
Again, as in making reciprocal crosses between the same two species, there is occasionally a great difference in the result, so something analogous
occurs with dimorphic
plants; for a short-styled cowslip yields more seed when fertilised by the long-styled
form, and less
seed when fertilised by its own form, than does a long-styled cowslip when fertilised in the two corresponding methods.
In all these respects
the
forms of the same undoubted species when illegitimately united behave in exactly the same manner as do two distinct species when crossed.
This led me carefully to observe during four years many seedlings, raised from several illegitimate unions.
The chief result is that these illegitimate plants, as they may be called, are not fully fertile.
It is possible to raise form
dimorphic species, both long-styled and short-styled illegitimate plants, and from trimorphic plants all three illegitimate forms;
so that
these
can be
properly united in a legitimate manner.
When this is done, there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilised.
But such is not the case;
they
are all infertile, but
in various degrees; some being so utterly and incurably sterile that they did not yield during four seasons a single seed or even a
seed-capsule.
These
illegitimate plants, which are so sterile, although
united with each other in a legitimate manner, may be strictly compared with hybrids
when crossed
inter
se,
and we all know how sterile these latter generally are.
When
on the other hand
a hybrid is crossed with either pure parent-species, the sterility is usually much lessened:
and so it is when an illegitimate plant is fertilised by a legitimate plant.
In the same manner as the sterility of hybrids does not always run parallel with the difficulty of making the first cross between its
two parent-species, so the sterility of certain illegitimate plants was unusually great, whilst the sterility of the union from which they were derived was by no means great.
With hybrids raised from the same seed-capsule the degree of sterility is innately variable, so it is in a marked manner with illegitimate plants.
Lastly, many hybrids are profuse and persistent flowerers, whilst other and more sterile hybrids produce few flowers, and are weak, miserable dwarfs; exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants.
Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids.
It is hardly an exaggeration to maintain that the former
are hybrids, but
produced within the limits of the same species by the improper union of certain forms, whilst ordinary hybrids are produced from an improper union between
so-called distinct species.
We have also already seen that there is the closest similarity in all respects between first illegitimate unions and first crosses between distinct species.
All
this
will perhaps be made more fully apparent by an illustration:
we may suppose that a botanist found two well-marked varieties (and such occur) of the long-styled form of the trimorphic Lythrum salicaria, and that he determined to try by crossing whether they were specifically distinct.
He would find that they yielded only about one-fifth of the proper number of seed, and that they behaved in all the other above specified respects as if they had been two distinct species.
But to make the case sure, he would raise plants from his supposed hybridised seed, and he would find that the seedlings were miserably dwarfed and utterly sterile, and that they behaved in all other respects like ordinary hybrids.
He might then maintain that he had actually proved, in accordance with the common view, that his two varieties were as good and as distinct species as any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are of
importance,
because they show us, firstly,
that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we are thus led to infer, as previously remarked,
that there must be
some unknown law or bond
connecting
the infertility both
of illegitimate unions and of first crosses, with the infertility
of their illegitimate and hybrid offspring;
thirdly, because we find, and this seems to me of especial im-
portance,
that two or three forms of the same species may exist and may differ in no respect,
except
in certain characters in their reproductive organs,—such as in the relative lengths of the stamens and pistils, in the size, form, and colour of the pollen-grains, in the structure of the stigma, and in the number and size of the seeds. |
|
| ↑ 4 blocks not present in 1859 1860 1861 1866 1872; present in 1869 |
| With, dimorphic plants, the unions between the two distinct forms are alone quite fertile, and produce quite fertile offspring, whilst unions between individuals belonging to the same form are more or less sterile; so that the result is exactly the reverse of what occurs with distinct species.
With dimorphic plants the resultant sterility is quite independent of any difference in general structure or constitution, for it arises from the union of individuals belonging not only to the same species, but to the same form.
It must, therefore, depend on the nature of the sexual elements, which are so adapted to each other, that the male and female elements occurring in the same form do not suit each other, whilst those occurring in the two distinct forms are mutually suited to each other.
From these considerations, it seems probable that the sterility of distinct species when crossed, and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any general difference in structure or constitution.
|
|
| ↑ 3 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| For we must remember that it is the union of the sexual elements of individuals of the same form, for instance, of two long-styled forms, which results in sterility; whilst it is the union of the sexual elements proper to two distinct forms which is fertile.
Hence the case appears at first sight exactly the reverse of what occurs, in the ordinary unions of the individuals of the same species and with crosses between distinct species.
It is, however, doubtful whether this is really so; but I will not enlarge on this obscure subject.
|
|
| ↑ 2 blocks not present in 1859 1860 1861 1869 1872; present in 1866 |
| With these differences and with no others, either in organisation or constitution, between the several forms, which are all hermaphrodites, we find that their illegitimate unions and their illegitimate progeny are more or less sterile, and closely resemble in a whole series of relations the first unions and the hybrid offspring of distinct species.
We are thus led to infer that the sterility of species when crossed and of their hybrid progeny is likewise in all probability exclusively due to similar differences confined to their reproductive systems.
|
|
| ↑ 1 blocks not present in 1859 1860 1861 1866 1869; present in 1872 |
| We may, however, infer as probable from the consideration of dimorphic and trimorphic plants, that the sterility of distinct species when crossed and of their hybrid progeny, depends exclusively on the nature of their sexual elements, and not on any difference in their structure or general constitution.
|
|
| →
crossed,
1859 |
|
crossed
,
1860 1861 |
|
Crossed,
1866 1869 |
|
Crossed
,
1872 |
|
| →
offspring
.—
1859 1860 1861 |
|
Offspring
. 1866 1869 |
|
Offspring
,
not universal
. 1872 |
|
| → a most forcible 1859 1860 1861 1866 |
| an overwhelming 1869 1872 |
|
