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22 chapters
PREFACE.
PREFACE.
In the Study of Evolution progress had well-nigh stopped. The more vigorous, perhaps also the more prudent, had left this field of science to labour in others where the harvest is less precarious or the yield more immediate. Of those who remained some still struggled to push towards truth through the jungle of phenomena: most were content supinely to rest on the great clearing Darwin made long since. Such was our state when two years ago it was suddenly discovered that an unknown man, Gregor Joh
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THE PROBLEMS OF HEREDITY AND THEIR SOLUTION3.
THE PROBLEMS OF HEREDITY AND THEIR SOLUTION3.
An exact determination of the laws of heredity will probably work more change in man’s outlook on the world, and in his power over nature, than any other advance in natural knowledge that can be clearly foreseen. There is no doubt whatever that these laws can be determined. In comparison with the labour that has been needed for other great discoveries we may even expect that the necessary effort will be small. It is rather remarkable that while in other branches of physiology such great progress
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Selection of the Experimental Plants.
Selection of the Experimental Plants.
The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used, and thus in the case before us it cannot be immaterial what plants are subjected to experiment and in what manner such experiments are conducted. The selection of the plant group which shall serve for experiments of this kind must be made with all possible care if it be desired to avoid from the outset every risk of questionable results. The experimental plants must necessar
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Division and Arrangement of the Experiments.
Division and Arrangement of the Experiments.
If two plants which differ constantly in one or several characters be crossed, numerous experiments have demonstrated that the common characters are transmitted unchanged to the hybrids and their progeny; but each pair of differentiating characters, on the other hand, unite in the hybrid to form a new character, which in the progeny of the hybrid is usually variable. The object of the experiment was to observe these variations in the case of each pair of differentiating characters, and to deduce
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The Forms of the Hybrids.31
The Forms of the Hybrids.31
Experiments which in previous years were made with ornamental plants have already afforded evidence that the hybrids, as a rule, are not exactly intermediate between the parental species. With some of the more striking characters, those, for instance, which relate to the form and size of the leaves, the pubescence of the several parts, &c., the intermediate, indeed, was nearly always to be seen; in other cases, however, one of the two parental characters was so preponderant that it was d
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The First Generation [Bred] from the Hybrids.
The First Generation [Bred] from the Hybrids.
In this generation there reappear, together with the dominant characters, also the recessive ones with their full peculiarities, and this occurs in the definitely expressed average proportion of three to one, so that among each four plants of this generation three display the dominant character and one the recessive. This relates without exception to all the characters which were embraced in the experiments. The angular wrinkled form of the seed, the green colour of the albumen, the white colour
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The Second Generation [Bred] from the Hybrids.
The Second Generation [Bred] from the Hybrids.
Those forms which in the first generation maintain the recessive character do not further vary in the second generation as regards this character; they remain constant in their offspring. It is otherwise with those which possess the dominant character in the first generation [bred from the hybrids]. Of these two -thirds yield offspring which display the dominant and recessive characters in the proportion of 3 to 1, and thereby show exactly the same ratio as the hybrid forms, while only one -thir
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The Subsequent Generations [Bred] from the Hybrids.
The Subsequent Generations [Bred] from the Hybrids.
The proportions in which the descendants of the hybrids develop and split up in the first and second generations presumably hold good for all subsequent progeny. Experiments 1 and 2 have already been carried through six generations, 3 and 7 through five, and 4, 5, and 6 through four, these experiments being continued from the third generation with a small number of plants, and no departure from the rule has been perceptible. The offspring of the hybrids separated in each generation in the ratio
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The Offspring of Hybrids in which Several Differentiating Characters are Associated.
The Offspring of Hybrids in which Several Differentiating Characters are Associated.
In the experiments above described plants were used which differed only in one essential character 36 . The next task consisted in ascertaining whether the law of development discovered in these applied to each pair of differentiating characters when several diverse characters are united in the hybrid by crossing. As regards the form of the hybrids in these cases, the experiments showed throughout that this invariably more nearly approaches to that one of the two parental plants which possesses
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The Reproductive Cells of Hybrids.
The Reproductive Cells of Hybrids.
The results of the previously described experiments induced further experiments, the results of which appear fitted to afford some conclusions as regards the composition of the egg and pollen cells of hybrids. An important matter for consideration is afforded in Pisum by the circumstance that among the progeny of the hybrids constant forms appear, and that this occurs, too, in all combinations of the associated characters. So far as experience goes, we find it in every case confirmed that consta
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Experiments with Hybrids of other Species of Plants.
Experiments with Hybrids of other Species of Plants.
It must be the object of further experiments to ascertain whether the law of development discovered for Pisum applies also to the hybrids of other plants. To this end several experiments were recently commenced. Two minor experiments with species of Phaseolus have been completed, and may be here mentioned. An experiment with Phaseolus vulgaris and Phaseolus nanus gave results in perfect agreement. Ph. nanus had together with the dwarf axis simply inflated green pods. Ph. vulgaris had, on the oth
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Concluding Remarks.
Concluding Remarks.
It can hardly fail to be of interest to compare the observations made regarding Pisum with the results arrived at by the two authorities in this branch of knowledge, Kölreuter and Gärtner, in their investigations. According to the opinion of both, the hybrids in outer appearance present either a form intermediate between the original species, or they closely resemble either the one or the other type, and sometimes can hardly be discriminated from it. From their seeds usually arise, if the fertil
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ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION
ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION
By G. Mendel. ( Communicated to the Meeting 9 June, 1869 50 . ) Although I have already undertaken many experiments in fertilisation between species of Hieracium , I have only succeeded in obtaining the following 6 hybrids, and only from one to three specimens of them. The difficulty of obtaining a larger number of hybrids is due to the minuteness of the flowers and their peculiar structure. On account of this circumstance it was seldom possible to remove the anthers from the flowers chosen for
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Introductory.
Introductory.
On the rediscovery and confirmation of Mendel’s Law by de Vries, Correns, and Tschermak two years ago, it became clear to many naturalists, as it certainly is to me, that we had found a principle which is destined to play a part in the Study of Evolution comparable only with the achievement of Darwin—that after the weary halt of forty years we have at last begun to march. If we look back on the post-Darwinian period we recognize one notable effort to advance. This effort—fruitful as it proved, m
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I. The Mendelian Principle of Purity of Germ-Cells and the Laws of Heredity Based on Ancestry.
I. The Mendelian Principle of Purity of Germ-Cells and the Laws of Heredity Based on Ancestry.
Professor Weldon’s article is entitled “Mendel’s Laws of Alternative Inheritance in Peas.” This title expresses the scope of Mendel’s work and discovery none too precisely and even exposes him to distinct misconception. To begin with, it says both too little and too much. Mendel did certainly determine Laws of Inheritance in peas—not precisely the laws Professor Weldon has been at the pains of drafting, but of that anon. Having done so, he knew what his discovery was worth. He saw, and rightly,
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II. Mendel and the Critic’s Version of him.
II. Mendel and the Critic’s Version of him.
The “Law of Dominance.” I proceed to the question of dominance which Professor Weldon treats as a prime issue, almost to the virtual concealment of the great fact of gametic purity. Cross-breds in general, AB and BA , named above, may present many appearances. They may all be indistinguishable from A , or from B ; some may appear A ’s and some B ’s; they may be patchworks of both; they may be blends presenting one or many grades between the two; and lastly they may have an appearance special to
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III. The Facts in regard to Dominance of Characters in Peas.
III. The Facts in regard to Dominance of Characters in Peas.
Professor Weldon refers to no experiments of his own and presumably has made none. Had he done so he would have learnt many things about dominance in peas, whether of the yellow cotyledon-colour or of the round form, that might have pointed him to caution. In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to send to the Cambridge Botanic Garden on my behalf a set of samples of the varieties of Pisum and Phaseolus , an exhibit of which had greatly interested me at the Paris
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IV. Professor Weldon’s collection of “Other Evidence concerning Dominance in Peas.”
IV. Professor Weldon’s collection of “Other Evidence concerning Dominance in Peas.”
A. In regard to cotyledon colour: Preliminary. I have been at some pains to show how the contradictory results, no doubt sometimes occurring, on which Professor Weldon lays such stress, may be comprehended without any injury to Mendel’s main conclusions. This excursion was made to save trouble with future discoverers of exceptions, though the existence of such facts need scarcely disturb many minds. As regards the dominance of yellow cotyledon-colour over green the whole number of genuine unconf
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V. Professor Weldon’s quotations from Laxton.
V. Professor Weldon’s quotations from Laxton.
In support of his conclusions Professor Weldon adduces two passages from Laxton, some of whose testimony we have just considered. This further evidence of Laxton is so important that I reproduce it in full. The first passage, published in 1866, is as follows:— “The results of experiments in crossing the Pea tend to show that the colour of the immediate offspring or second generation sometimes follows that of the female parent, is sometimes intermediate between that and the male parent, and is so
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VI. The Argument Built on Exceptions.
VI. The Argument Built on Exceptions.
So much for the enormous advance that the Mendelian principles already permit us to make. But what does Professor Weldon offer to substitute for all this? Nothing. Professor Weldon suggests that a study of ancestry will help us. Having recited Tschermak’s exceptions and the great irregularities seen in the Telephone group, he writes: “Taking these results together with Laxton’s statements, and with the evidence afforded by the Telephone group of hybrids, I think we can only conclude that segrega
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VII. The question of absolute purity of germ-cells.
VII. The question of absolute purity of germ-cells.
But let us go back to the cases of defective “purity” and consider how the laws of ancestry stand in regard to them. It appears from the facts almost certain that purity may sometimes be wanting in a character which elsewhere usually manifests it. Here we approach a question of greater theoretical consequence to the right apprehension of the part borne by Mendelian principles in the physiology of heredity. We have to consider the question whether the purity of the gametes in respect of one or ot
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Conclusion.
Conclusion.
I trust what I have written has convinced the reader that we are, as was said in opening, at last beginning to move. Professor Weldon declares he has “no wish to belittle the importance of Mendel’s achievement”; he desires “simply to call attention to a series of facts which seem to him to suggest fruitful lines of inquiry.” In this purpose I venture to assist him, for I am disposed to think that unaided he is—to borrow Horace Walpole’s phrase—about as likely to light a fire with a wet dish-clou
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