Part 9

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.

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, that he had found a principle which _must_ govern a wide area of phenomena. He entitles his paper therefore “_Versuche über Pflanzen-Hybriden_,” or, Experiments in Plant-Hybridisation.

Nor did Mendel start at first with any particular intention respecting Peas. He tells us himself that he wanted to find the laws of inheritance in _hybrids_, which he suspected were definite, and that after casting about for a suitable subject, he found one in peas, for the reasons he sets out.

In another respect the question of title is much more important. By the introduction of the word “Alternative” the suggestion is made that the Mendelian principle applies peculiarly to cases of “alternative” inheritance. Mendel himself makes no such limitation in his earlier paper, though perhaps by rather remote implication in the second, to which the reader should have been referred. On the contrary, he wisely abstains from prejudicial consideration of unexplored phenomena.

* * * * *

To understand the significance of the word “alternative” as introduced by Professor Weldon we must go back a little in the history of these studies. In the year 1897 Galton formally announced the Law of Ancestral Heredity referred to in the _Introduction_, having previously “stated it briefly and with hesitation” in _Natural Inheritance_, p. 134. In 1898 Professor Pearson published his modification and generalisation of Galton’s Law, introducing a correction of admitted theoretical importance, though it is not in question that the principle thus restated is fundamentally not very different from Galton’s[54]. _It is an essential part of the Galton-Pearson Law of Ancestral Heredity that in calculating the probable structure of each descendant the structure of each several ancestor must be brought to account._

[54] I greatly regret that I have not a precise understanding of the basis of the modification proposed by Pearson. His treatment is in algebraical form and beyond me. Nevertheless I have every confidence that the arguments are good and the conclusion sound. I trust it may not be impossible for him to provide the non-mathematical reader with a paraphrase of his memoir. The arithmetical differences between the original and the modified law are of course clear.

Professor Weldon now tells us that these two papers of Galton and of Professor Pearson have “given us an expression for the effects of _blended_ inheritance which seems likely to prove generally applicable, though the constants of the equations which express the relation between divergence from the mean in one generation, and that in another, may require modification in special cases. Our knowledge of _particulate_ or mosaic inheritance, and of _alternative_ inheritance, is however still rudimentary, and there is so much contradiction between the results obtained by different observers, that the evidence available is difficult to appreciate.”

But Galton stated (p. 401) in 1897 that his statistical law of heredity “appears to be universally applicable to bi-sexual descent.” Pearson in re-formulating the principle in 1898 made no reservation in regard to “alternative” inheritance. On the contrary he writes (p. 393) that “if Mr Galton’s law can be firmly established, _it is a complete solution, at any rate to a first approximation, of the whole problem of heredity_,” and again (p. 412) that “it is highly probable that it [this law] is the simple descriptive statement which brings into a single focus all the complex lines of hereditary influence. If Darwinian evolution be natural selection combined with _heredity_, then the single statement which embraces the whole field of heredity must prove almost as epoch-making as the law of gravitation to the astronomer[55].”

[55] I have searched Professor Pearson’s paper in vain for any considerable reservation regarding or modification of this general statement. Professor Pearson enuntiates the law as “only correct on certain limiting hypotheses,” but he declares that of these the most important is “the absence of reproductive selection, i.e. the negligible correlation of fertility with the inherited character, and the absence of sexual selection.” The case of in-and-in breeding is also reserved.

As I read there comes into my mind that other fine passage where Professor Pearson warns us

“There is an insatiable desire in the human breast to resume in some short formula, some brief statement, the facts of human experience. It leads the savage to ‘account’ for all natural phenomena by deifying the wind and the stream and the tree. It leads civilized man, on the other hand, to express his emotional experience in works of art, and his physical and mental experience in the formulae or so-called laws of science[56].”

[56] K. Pearson, _Grammar of Science_, 2nd ed. 1900, p. 36.

No naturalist who had read Galton’s paper and had tried to apply it to the facts he knew could fail to see that here was a definite advance. We could all perceive phenomena that were in accord with it and there was no reasonable doubt that closer study would prove that accord to be close. It was indeed an occasion for enthusiasm, though no one acquainted with the facts of experimental breeding could consider the suggestion of universal application for an instant.

But two years have gone by, and in 1900 Pearson writes[57] that the values obtained from the Law of Ancestral Heredity

[57] _Grammar of Science_, 2nd ed. 1900, p. 480.

“seem to fit the observed facts fairly well in the case of _blended_ inheritance. In other words we have a certain amount of evidence in favour of the conclusion: _That whenever the sexes are equipotent, blend their characters and mate pangamously, all characters will be inherited at the same rate_,”

or, again in other words, that the Law of Ancestral Heredity after the glorious launch in 1898 has been home for a complete refit. The top-hamper is cut down and the vessel altogether more manageable; indeed she looks trimmed for most weathers. Each of the qualifications now introduced wards off whole classes of dangers. Later on (pp. 487–8) Pearson recites a further list of cases regarded as exceptional. “All characters will be inherited at the same rate” might indeed almost be taken to cover the results in Mendelian cases, though the mode by which those results are arrived at is of course wholly different.

Clearly we cannot speak of the Law of Gravitation now. Our Tycho Brahe and our Kepler, with the yet more distant Newton, are appropriately named as yet to come[58].

[58] _Phil. Trans._ 1900, vol. 195, A, p. 121.

But the truth is that even in 1898 such a comparison was scarcely happy. Not to mention moderns, these high hopes had been finally disposed of by the work of the experimental breeders such as Kölreuter, Knight, Herbert, Gärtner, Wichura, Godron, Naudin, and many more. To have treated as non-existent the work of this group of naturalists, who alone have attempted to solve the problems of heredity and species--Evolution, as we should now say--by the only sound method--_experimental breeding_--to leave out of consideration almost the whole block of evidence collected in _Animals and Plants_--Darwin’s finest legacy as I venture to declare--was unfortunate on the part of any exponent of Heredity, and in the writings of a professed naturalist would have been unpardonable. But even as modified in 1900 the Law of Ancestral Heredity is heavily over-sparred, and any experimental breeder could have increased Pearson’s list of unconformable cases by as many again.

But to return to Professor Weldon. He now repeats that the Law of Ancestral Heredity seems likely to prove generally applicable to _blended_ inheritance, but that the case of _alternative_ inheritance is for the present reserved. We should feel more confidence in Professor Weldon’s exposition if he had here reminded us that the special case which fitted Galton’s Law so well that it emboldened him to announce that principle as apparently “universally applicable to bi-sexual descent” was one of _alternative_ inheritance--namely the coat-colour of Basset-hounds. Such a fact is, to say the least, ominous. Pearson, in speaking (1900) of this famous case of Galton’s, says that these phenomena of alternative inheritance must be treated separately (from those of blended inheritance)[59], and for them he deduces a proposed “_law of reversion_,” based of course on ancestry. He writes, “In both cases we may speak of a law of ancestral heredity, but the first predicts the probable character of the individual produced by a given ancestry, while the second tells us the percentages of the total offspring which on the average revert to each ancestral type[60].”

[59] “If this be done, we shall, I venture to think, keep not only our minds, but our points for observation, clearer; and further, the failure of Mr Galton’s statement in the one case will not in the least affect its validity in the other.” Pearson (32), p. 143.

[60] _Grammar of Science_, 1900, p. 494. See also Pearson, _Proc. Roy. Soc._ 1900, LXVI. pp. 142–3.

With the distinctions between the original Law of Ancestral Heredity, the modified form of the same law, and the Law of Reversion, important as all these considerations are, we are not at present concerned.

For the Mendelian principle of heredity asserts a proposition absolutely at variance with all the laws of ancestral heredity, however formulated. In those cases to which it applies strictly, this principle declares that the cross-breeding of parents _need_ not diminish the purity of their germ-cells or consequently the purity of their offspring. When in such cases individuals bearing opposite characters, _A_ and _B_, are crossed, the germ-cells of the resulting cross-bred, _AB_, are each to be bearers either of character A or of character _B_, not both.

Consequently when the cross-breds breed either together or with the pure forms, individuals will result of the forms _AA_, _AB_, _BA_, _BB_[61]. Of these the forms _AA_ and _BB_, formed by the union of similar germs, are stated to be as pure as if they had had no cross in their pedigree, and henceforth their offspring will be no more likely to depart from the _A_ type or the _B_ type respectively, than those of any other originally pure specimens of these types.

[61] On an average of cases, in equal numbers, as Mendel found.

Consequently in such examples it is _not_ the fact that each ancestor must be brought to account as the Galton-Pearson Law asserts, and we are clearly dealing with a physiological phenomenon not contemplated by that Law at all.