Origin of Species Variorum

→ may conclude 1869 1872

are thus led to infer, as previously remarked, 1866

→ bond which 1869 1872

law or bond 1866

→ with that 1869 1872

and of first crosses, with the infertility 1866

→ offspring, and we are led to extend the same view to first crosses and hybrids; 1869 1872

and hybrid offspring; 1866

→ whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when united 1872

except 1866 1869

→ certain ways. 1872

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. 1866

their reproductive organs, and yet be sterile when united in certain ways. 1869

↑ 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.

↑ 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.

→ OMIT 1869 1872

between the same two species, 1866

→ OMIT 1872

in 1866

of the second species in 1869

→ concluded 1872

came on general grounds to the same conclusion, namely, 1866

came to this same conclusion, namely, 1869

↑ 10 blocks not present in 1859 1860 1861 1869 1872; present in 1866

Finally, we are naturally led to inquire for what useful end have plants been rendered reciprocally dimorphic and trimorphic? A wide-spread analogy clearly gives us the answer as far as the immediate cause is concerned, namely, to prevent the pollen of each flower acting on the stigma of that flower. We see this effected in a host of flowers by the most curious mechanical contrivances, as I have shown with Orchids, and as could be shown with many plants of many other orders. There are also numerous plants, called dicho-gamous by C. K. Sprengel, in which the pollen and stigma are never mature at the same time, so that these plants can never fertilise themselves. There are many flowers, which, though they have their stigmas and pollen mature together, and which do not present any obstacle to self-impregnation, yet nevertheless are almost always fertilised by surrounding varieties when growing in the vicinity, as shown by the character of their seedlings. Then, again, we have many flowers with separated sexes borne on distinct plants, or on the same, which inevitably prevents self-fertilisation. Lastly, in accordance with the great principle prevailing throughout nature, of the same end being gained by the most diversified means, we find in dimorphic and trimorphic plants, in which self-fertilisation is not checked by any of the above-specified means, that this has been effected by the pollen of each flower, and consequently of all the flowers of the same form, having been rendered more or less impotent on their own stigmas; so that its action is easily and wholly obliterated by pollen habitually brought by insects from other individuals and forms of the same species. In searching for the cause of dimorphism and trimorphism in plants, we may, in my opinion, safely go one step further, and conclude that the pollen has been prevented acting on the stigma of the same flower, in order to give vigour to the offspring by leading to the union of two distinct individuals. But on this view it is not a little remarkable that the end has been gained, in the case of dimorphic and trimorphic plants, at the expense of all the plants of the same form being rendered more or less sterile when united, and producing more or less sterile offspring. With respect to the steps by which it is probable that plants have been rendered dimorphic and trimorphic, want of space prevents my entering on the subject; but I will add that there is no special difficulty in this having been effected through variability, through the good gained by the prepotency of one sort of pollen over another, and through the accumulative action of natural selection.

→ Crossed , 1872

crossed, 1859

crossed , 1860 1861

Crossed, 1866 1869

→ Offspring , not universal . 1872

offspring .— 1859 1860 1861

Offspring . 1866 1869

→ an overwhelming 1869 1872

a most forcible 1859 1860 1861 1866

→ OMIT 1869 1872

and that there must be some error in all the foregoing remarks, 1859 1860 1861 1866

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

....

importance,

important,

because they show us,

firstly,

first,

that the physiological test of lessened fertility, both in first crosses and in hybrids, is no safe criterion of specific distinction; secondly, because we →may conclude that there

must be

some unknown →bond which

connecting

connects

the infertility

both

....

of illegitimate unions →with that of their illegitimate →offspring, and we are led to extend the same view to first crosses and hybrids; thirdly, because we find, and this seems to me of especial

im-

....

portance,

importance,

that two or three forms of the same species may exist and may differ in no

respect,

respect

→whatever, either in structure or in constitution, relatively to external conditions, and yet be sterile when united in →certain ways. ↑ 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. ↑ 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. We are

indeed

also

led to

his

this

same conclusion

from

considering reciprocal

crosses

crosses,

→OMIT in which the male of one

cannot

species cannot

be united, or can be united with great difficulty, with the female of

the other

a second

species, whilst the converse cross can be effected with perfect

facility;

for

this

difference

the

facility

making

reciprocal

crosses

and

the

fertility

their

offspring

must

attributed

either

the

male

the

female

element

the

one

first

species

having

been

differentiated,

with

reference

the

other

~~OMIT~~

....

sexual

....

element,

elements

→OMIT

higher

degree

than

the

second species.

converse case.

....

That excellent observer, Gärtner, likewise →concluded that species when crossed are sterile owing to differences confined to their reproductive systems. ↑

Fertility

Varieties

when

→ Crossed ,

and

their

Mongrel

→ Offspring , not universal .

It may be urged, as →an overwhelming

argu- ment,

argument,

that there must be some essential distinction between species and varieties, →OMIT inasmuch as

varieties,

the latter,

however much they may differ