Chapter V: Introduction (4)

Gärtner found by repeated experiments that the respective period of transformation varies in many species, so that frequently a species _A_ can be transformed into a species _B_ a generation sooner than can species _B_ into species _A_. He deduces therefrom that Kölreuter’s opinion can hardly be maintained that “the two natures in hybrids are perfectly in equilibrium.” It appears, however, that Kölreuter does not merit this criticism, but that Gärtner rather has overlooked a material point, to which he himself elsewhere draws attention, viz. that “it depends which individual is chosen for further transformation.” Experiments which in this connection were carried out with two species of _Pisum_ demonstrated that as regards the choice of the fittest individuals for the purpose of further fertilisation it may make a great difference which of two species is transformed into the other. The two experimental plants differed in five characters, while at the same time those of species _A_ were all dominant and those of species _B_ all recessive. For mutual transformation _A_ was fertilised with pollen of _B_, and _B_ with pollen of _A_, and this was repeated with both hybrids the following year. With the first experiment _B_/_A_ there were eighty-seven plants available in the third year of experiment for the selections of individuals for further crossing, and these were of the possible thirty-two forms; with the second experiment _A_/_B_ seventy-three plants resulted, which _agreed throughout perfectly in habit with the pollen parent_; in their internal composition, however, they must have been just as varied as the forms of the other experiment. A definite selection was consequently only possible with the first experiment; with the second some plants selected at random had to be excluded. Of the latter only a portion of the flowers were crossed with the _A_ pollen, the others were left to fertilise themselves. Among each five plants which were selected in both experiments for fertilisation there agreed, as the following year’s culture showed, with the pollen parent:--

1st Experiment. 2nd Experiment.
2 plants -- in all characters
3 " -- " 4 "
-- 2 plants " 3 "
-- 2 " " 2 "
-- 1 plant " 1 character

In the first experiment, therefore, the transformation was completed; in the second, which was not continued further, two more fertilisations would probably have been required.

Although the case may not frequently occur that the dominant characters belong exclusively to one or the other of the original parent plants, it will always make a difference which of the two possesses the majority. If the pollen parent shows the majority, then the selection of forms for further crossing will afford a less degree of security than in the reverse case, which must imply a delay in the period of transformation, provided that the experiment is only considered as completed when a form is arrived at which not only exactly resembles the pollen plant in form, but also remains as constant in its progeny.

Gärtner, by the results of these transformation experiments, was led to oppose the opinion of those naturalists who dispute the stability of plant species and believe in a continuous evolution of vegetation. He perceives in the complete transformation of one species into another an indubitable proof that species are fixed within limits beyond which they cannot change. Although this opinion cannot be unconditionally accepted we find on the other hand in Gärtner’s experiments a noteworthy confirmation of that supposition regarding variability of cultivated plants which has already been expressed.

Among the experimental species there were cultivated plants, such as _Aquilegia atropurpurea_ and _canadensis_, _Dianthus caryophyllus_, _chinensis_, and _japonicus_, _Nicotiana rustica_ and _paniculata_, and hybrids between these species lost none of their stability after four or five generations[49].

[49] [The argument of these two last paragraphs appears to be that
though the general mutability of natural species might be doubtful,
yet among cultivated plants the transference of characters may be
accomplished, and may occur by integral steps until one species is
definitely “transformed” into the other.]

ON HIERACIUM-HYBRIDS OBTAINED BY ARTIFICIAL FERTILISATION

By G. Mendel.

(_Communicated to the Meeting 9 June, 1869[50]._)

[50] [Published in _Verh. naturf. Ver. Brünn, Abhandlungen_, VIII. 1869, p. 26, which appeared in 1870.]

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.

_H. Auricula_ ♀ × _H. aurantiacum_ ♂
_H. Auricula_ ♀ × _H. Pilosella_ ♂
_H. Auricula_ ♀ × _H. pratense_ ♂
_H. echioides_[51] ♀ × _H. aurantiacum_ ♂
_H. præaltum_ ♀ × _H. flagellare_ Rchb. ♂
_H. præaltum_ ♀ × _H. aurantiacum_ ♂

[51] The plant used in this experiment is not exactly the typical _H.
echioides_. It appears to belong to the series transitional to _H.
præaltum_, but approaches more nearly to _H. echioides_ and for this
reason was reckoned as belonging to the latter.

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 fertilisation without either letting pollen get on to the stigma or injuring the pistil so that it withered away. As is well known, the anthers are united to form a tube, which closely embraces the pistil. As soon as the flower opens, the stigma, already covered with pollen, protrudes. In order to prevent self-fertilisation the anther-tube must be taken out before the flower opens, and for this purpose the bud must be slit up with a fine needle. If this operation is attempted at a time when the pollen is mature, which is the case two or three days before the flower opens, it is seldom possible to prevent self-fertilisation; for with every care it is not easily possible to prevent a few pollen grains getting scattered and communicated to the stigma. No better result has been obtained hitherto by removing the anthers at an earlier stage of development. Before the approach of maturity the tender pistil and stigma are exceedingly sensitive to injury, and even if they are not actually injured, they generally wither and dry up after a little time if deprived of their protecting investments. I hope to obviate this last misfortune by placing the plants after the operation for two or three days in the damp atmosphere of a greenhouse. An experiment lately made with _H. Auricula_ treated in this way gave a good result.

To indicate the object with which these fertilisation experiments were undertaken, I venture to make some preliminary remarks respecting the genus _Hieracium_. This genus possesses such an extraordinary profusion of distinct forms that no other genus of plants can compare with it. Some of these forms are distinguished by special peculiarities and may be taken as type-forms of species, while all the rest represent intermediate and transitional forms by which the type-forms are connected together. The difficulty in the separation and delimitation of these forms has demanded the close attention of the experts. Regarding no other genus has so much been written or have so many and such fierce controversies arisen, without as yet coming to a definite conclusion. It is obvious that no general understanding can be arrived at, so long as the value and significance of the intermediate and transitional forms is unknown.

Regarding the question whether and to what extent hybridisation plays a part in the production of this wealth of forms, we find very various and conflicting views held by leading botanists. While some of them maintain that this phenomenon has a far-reaching influence, others, for example, Fries, will have nothing to do with hybrids in _Hieracia_. Others take up an intermediate position; and while granting that hybrids are not rarely formed between the species in a wild state, still maintain that no great importance is to be attached to the fact, on the ground that they are only of short duration. The [suggested] causes of this are partly their restricted fertility or complete sterility; partly also the knowledge, obtained by experiment, that in hybrids self-fertilisation is always prevented if pollen of one of the parent-forms reaches the stigma. On these grounds it is regarded as inconceivable that _Hieracium_ hybrids can constitute and maintain themselves as fully fertile and constant forms when growing near their progenitors.

The question of the origin of the numerous and constant intermediate forms has recently acquired no small interest since a famous _Hieracium_ specialist has, in the spirit of the Darwinian teaching, defended the view that these forms are to be regarded as [arising] from the transmutation of lost or still existing species.

From the nature of the subject it is clear that without an exact knowledge of the structure and fertility of the hybrids and the condition of their offspring through several generations no one can undertake to determine the possible influence exercised by hybridisation over the multiplicity of intermediate forms in _Hieracium_. The condition of the _Hieracium_ hybrids in the range we are concerned with must necessarily be determined by experiments; for we do not possess a complete theory of hybridisation, and we may be led into erroneous conclusions if we take rules deduced from observation of certain other hybrids to be Laws of hybridisation, and try to apply them to _Hieracium_ without further consideration. If by the experimental method we can obtain a sufficient insight into the phenomenon of hybridisation in _Hieracium_, then by the help of the experience which has been collected respecting the structural relations of the wild forms, a satisfactory judgment in regard to this question may become possible.

Thus we may express the object which was sought after in these experiments. I venture now to relate the very slight results which I have as yet obtained with reference to this object.

1. Respecting the structure of the hybrids, we have to record the striking phenomenon that the forms hitherto obtained by similar fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three individuals, while as to the remainder only one of each has been obtained.

If we compare the individual characters of the hybrids with the corresponding characters of the two parent types, we find that they sometimes present intermediate structures, but are sometimes so near to one of the parent characters that the [corresponding] character of the other has receded considerably or almost evades observation. So, for instance, we see in one of the two forms of _H. Auricula_ ♀ x _H. aurantiacum_ ♂ pure yellow disc-florets; only the petals of the marginal florets are on the outside tinged with red to a scarcely noticeable degree: in the other on the contrary the colour of these florets comes very near to _H. aurantiacum_, only in the centre of the disc the orange red passes into a deep golden-yellow. This difference is noteworthy, for the flower-colour in _Hieracium_ has the value of a constant character. Other similar cases are to be found in the leaves, the peduncles, &c.

If the hybrids are compared with the parent types as regards the sum total of their characters, then the two forms of _H. præaltum_ ♀ x _H. aurantiacum_ ♂ constitute approximately intermediate forms which do not agree in certain characters. On the contrary in _H. Auricula_ ♀ x _H. aurantiacum_ ♂ and in _H. Auricula_ ♀ x _H. pratense_ ♂ we see the forms widely divergent, so that one of them is nearer to the one and the other to the other parental type, while in the case of the last-named hybrid there is still a third which is almost precisely intermediate between them.

The conviction is then forced on us that we have here only single terms in an unknown series which may be formed by the direct action of the pollen of one species on the egg-cells of another.

2. With a single exception the hybrids in question form seeds capable of germination. _H. echioides_ ♀ x _H. aurantiacum_ ♂ may be described as fully fertile; _H. præaltum_ ♀ x _H. flagellare_ ♂ as fertile; _H. præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. pratense_ ♂ as partially fertile; _H. Auricula_ ♀ x _H. Pilosella_ ♂ as slightly fertile, and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ as unfertile. Of the two forms of the last named hybrid, the red-flowered one was completely sterile, but from the yellow-flowered one a single well-formed seed was obtained. Moreover it must not pass unmentioned that among the seedlings of the partially fertile hybrid _H. præaltum_ ♀ x _H. aurantiacum_ ♂ there was one plant which possessed full fertility.

[3.] As yet the offspring produced by self-fertilisation of the hybrids have not varied, but agree in their characters both with each other and with the hybrid plant from which they were derived.

From _H. præaltum_ ♀ x _H. flagellare_ ♂ two generations have flowered; from _H. echioides_ ♀ x _H. aurantiacum_ ♂, _H. præaltum_ ♀ x _H. aurantiacum_ ♂, _H. Auricula_ ♀ x _H. Pilosella_ ♂ one generation in each case has flowered.

4. The fact must be declared that in the case of the fully fertile hybrid _H. echioides_ ♀ x _H. aurantiacum_ ♂ the pollen of the parent types was not able to prevent self-fertilisation, though it was applied in great quantity to the stigmas protruding through the anther-tubes when the flowers opened.

From two flower-heads treated in this way seedlings were produced resembling this hybrid plant. A very similar experiment, carried out this summer with the partially fertile _H. præaltum_ ♀ x _H. aurantiacum_ ♂ led to the conclusion that those flower-heads in which pollen of the parent type or of some other species had been applied to the stigmas, developed a notably larger number of seeds than those which had been left to self-fertilisation alone. The explanation of this result must only be sought in the circumstance that as a large part of the pollen-grains of the hybrid, examined microscopically, show a defective structure, a number of egg-cells capable of fertilisation do not become fertilised by their own pollen in the ordinary course of self-fertilisation.

It not rarely happens that in fully fertile species in the wild state the formation of the pollen fails, and in many anthers not a single good grain is developed. If in these cases seeds are nevertheless formed, such fertilisation must have been effected by foreign pollen. In this way hybrids may easily arise by reason of the fact that many forms of insects, notably the industrial Hymenoptera, visit the flowers of _Hieracia_ with great zeal and are responsible for the pollen which easily sticks to their hairy bodies reaching the stigmas of neighbouring plants.

From the few facts that I am able to contribute it will be evident the work scarcely extends beyond its first inception. I must express some scruple in describing in this place an account of experiments just begun. But the conviction that the prosecution of the proposed experiments will demand a whole series of years, and the uncertainty whether it will be granted to me to bring the same to a conclusion have determined me to make the present communication. By the kindness of Dr Nägeli, the Munich Director, who was good enough to send me species which were wanting, especially from the Alps, I am in a position to include a larger number of forms in my experiments. I venture to hope even next year to be able to contribute something more by way of extension and confirmation of the present account.

If finally we compare the described result, still very uncertain, with those obtained by crosses made between forms of _Pisum_, which I had the honour of communicating in the year 1865, we find a very real distinction. In _Pisum_ the hybrids, obtained from the immediate crossing of two forms, have in all cases the same type, but their posterity, on the contrary, are variable and follow a definite law in their variations. In _Hieracium_ according to the present experiments the exactly opposite phenomenon seems to be exhibited. Already in describing the _Pisum_ experiments it was remarked that there are also hybrids whose posterity do not vary, and that, for example, according to Wichura the hybrids of _Salix_ reproduce themselves like pure species. In _Hieracium_ we may take it we have a similar case. Whether from this circumstance we may venture to draw the conclusion that the polymorphism of the genera _Salix_ and _Hieracium_ is connected with the special condition of their hybrids is still an open question, which may well be raised but not as yet answered.

A DEFENCE OF MENDEL’S PRINCIPLES OF HEREDITY.

“_The most fertile men of science have made blunders, and their
consciousness of such slips has been retribution enough; it is only
their more sterile critics who delight to dwell too often and too
long on such mistakes._” BIOMETRIKA, 1901.

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, memorable as it must ever be--was that made by Galton when he enuntiated his Law of Ancestral Heredity, subsequently modified and restated by Karl Pearson. Formulated after long and laborious inquiry, this principle beyond question gives us an expression including and denoting many phenomena in which previously no regularity had been detected. But to practical naturalists it was evident from the first that there are great groups of facts which could not on any interpretation be brought within the scope of Galton’s Law, and that by no emendation could that Law be extended to reach them. The existence of these phenomena pointed to a different physiological conception of heredity. Now it is precisely this conception that Mendel’s Law enables us to form. Whether the Mendelian principle can be extended so as to include some apparently Galtonian cases is another question, respecting which we have as yet no facts to guide us, but we have certainly no warrant for declaring such an extension to be impossible.

Whatever answer the future may give to that question, it is clear from this moment that every case which obeys the Mendelian principle is removed finally and irretrievably from the operations of the Law of Ancestral Heredity.

At this juncture Professor Weldon intervenes as a professed exponent of Mendel’s work. It is not perhaps to a devoted partisan of the Law of Ancestral Heredity that we should look for the most appreciative exposition of Mendel, but some bare measure of care and accuracy in representation is demanded no less in justice to fine work, than by the gravity of the issue.

Professor Weldon’s article appears in the current number of _Biometrika_, Vol. I. Pt. II. which reached me on Saturday, Feb. 8. The paper opens with what purports to be a restatement of Mendel’s experiments and results. In this “restatement” a large part of Mendel’s experiments--perhaps the most significant--are not referred to at all. The perfect simplicity and precision of Mendel’s own account are destroyed; with the result that the reader of Professor Weldon’s paper, unfamiliar with Mendel’s own memoir, can scarcely be blamed if he fail to learn the essence of the discovery. Of Mendel’s conception of the hybrid as a distinct entity with characters proper to itself, apart from inheritance--the most novel thing in the whole paper--Professor Weldon gives no word. Upon this is poured an undigested mass of miscellaneous “facts” and statements from which the reader is asked to conclude, first, that a proposition attributed to Mendel regarding dominance of one character is not of “general”[52] application, and finally that “all work based on Mendel’s method” is “vitiated” by a “fundamental mistake,” namely “the neglect of ancestry[53].”

[52] The words “general” and “universal” appear to be used by
Professor Weldon as interchangeable. Cp. Weldon, p. 235 and
elsewhere, with Abstract given below.

[53] These words occur p. 252: “The fundamental mistake which
vitiates all work based upon Mendel’s method is the neglect of
ancestry, and the attempt to regard the whole effect upon offspring
produced by a particular parent, as due to the existence in the
parent of particular structural characters, &c.” As a matter of fact
the view indicated in these last words is especially repugnant to the
Mendelian principle, as will be seen.

To find a parallel for such treatment of a great theme in biology we must go back to those writings of the orthodox which followed the appearance of the “Origin of Species.”

On 17th December 1900 I delivered a Report to the Evolution Committee of the Royal Society on the experiments in Heredity undertaken by Miss E. R. Saunders and myself. This report has been offered to the Society for publication and will I understand shortly appear. In it we have attempted to show the extraordinary significance of Mendel’s principle, to point out what in his results is essential and what subordinate, the ways in which the principle can be extended to apply to a diversity of more complex phenomena--of which some are incautiously cited by Professor Weldon as conflicting facts--and lastly to suggest a few simple terms without which (or some equivalents) the discussion of such phenomena is difficult. Though it is impossible here to give an outline of facts and reasoning there set out at length, I feel that his article needs an immediate reply. Professor Weldon is credited with exceptional familiarity with these topics, and his paper is likely to be accepted as a sufficient statement of the case. Its value will only be known to those who have either worked in these fields themselves or have been at the trouble of thoughtfully studying the original materials.

The nature of Professor Weldon’s article may be most readily indicated if I quote the summary of it issued in a paper of abstracts sent out with Review copies of the Part. This paper was most courteously sent to me by an editor of _Biometrika_ in order to call my attention to the article on Mendel, a subject in which he knew me to be interested. The abstract is as follows.

“Few subjects have excited so much interest in the last year or two
as the laws of inheritance in hybrids. Professor W. F. R. Weldon
describes the results obtained by Mendel by crossing races of Peas
which differed in one or more of seven characters. From a study of
the work of other observers, and from examination of the ‘Telephone’
group of hybrids, the conclusion is drawn that Mendel’s results
do not justify any general statement concerning inheritance in
cross-bred Peas. A few striking cases of other cross-bred plants and
animals are quoted to show that the results of crossing cannot, as
Mendel and his followers suggest, be predicted from a knowledge of
the characters of the two parents crossed without knowledge of the
more remote ancestry.”

Such is the judgment a fellow-student passes on this mind

“_Voyaging through strange seas of thought alone._”

The only conclusion which most readers could draw from this abstract and indeed from the article it epitomizes, is that Mendel’s discovery so far from being of paramount importance, rests on a basis which Professor Weldon has shown to be insecure, and that an error has come in through disregard of the law of Ancestral Heredity. On examining the paper it is perfectly true that Professor Weldon is careful nowhere directly to question Mendel’s facts or his interpretation of them, for which indeed in some places he even expresses a mild enthusiasm, but there is no mistaking the general purpose of the paper. It must inevitably produce the impression that the importance of the work has been greatly exaggerated and that supporters of current views on Ancestry may reassure themselves. That this is Professor Weldon’s own conclusion in the matter is obvious. After close study of his article it is evident to me that Professor Weldon’s criticism is baseless and for the most part irrelevant, and I am strong in the conviction that the cause which will sustain damage from this debate is not that of Mendel.