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Mendel in Darwin's Shadow

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BY DAVID ALLEN

My research is largely ignored and I [illegible] momentous discovery will be lost.
I believe I have discovered the mechanism [illegible] the simple seed [illegible]
Could these [illegible] also operate in other [illegible] of life?
I fear for my soul because [illegible] may be heres[y]
[illegible] not in the seed of man [him]self?
[illegible] tell you everything upon my return.

  Yours with love and affection,
  Gregor Mendel


Gregor Mendel
Gregor Mendel
(1822–1884)
This fragment of a supposedly 'lost' letter by Gregor Mendel (see image above), is actually a photographic composition, created by an artist called 'Dr. Confuser.'1 The way the letter is shot at an odd angle (so that portions of it lie outside the frame of the picture), and an old nib pen and a number of dried peas lie on the page, makes some of the text illegible—leaving us only with these fragments to digest. The actual historical record we have for Gregor Mendel is nearly as fragmentary: a few letters and articles; some elliptical notes and pencil marks scribbled in books he read; scraps of information collected from people who knew him; very little else.

In his photocomposition, 'Dr. Confuser' offers us an imaginative speculation on how Mendel might have felt about his discoveries. Did he really fear for his ‘soul’ because his discoveries might be ‘heresy’? There is no historical evidence for this; but the photocomposition does not pretend to be any more than one person’s interpretation—a piece of fiction. (The fact the letter is written in English might give us a clue that it is fiction; but on the website where the photograph first appeared, the artist also tells us that he wrote it himself on a grocery sack, and the pen came from his wife's calligraphy set.) Given the paucity of the historical record on Mendel, all interpretations have to rely to a certain extent on imaginative speculation. But in all that has been written about him, speculation often presents itself as fact.


Creationist or Evolutionist?

There has long been an ideological fault line running through Mendel criticism, and it centres on his attitude to evolution. Hugo Iltis, Mendel's first biographer, championed him as an evolutionist, and claimed he was 'suspected as a Darwinist not without reason' (1951, p. 30). On the other hand, Oswald Richter heralded him as the 'most important witness against the basic teachings of the Darwinian theory' (1943, p. 262). Others, too, have since claimed him as an anti-Darwinian. In 1996, B. E. Bishop argued (p. 205) that the Pisum paper was not only 'antievolutionary in content', but also that it was 'specifically written in contradiction of Darwin's book, On the Origin of Species'.

Works Cited
Biology Dictionary >>
Famous Biologists >>

The dispute over Mendel, in fact, is being fought in the shadow of another battle, over Darwinism. The debate has recently taken a more sinister twist, as proponents of Intelligent Design (ID) theory have attempted to appropriate Mendel. Steve Fuller openly declares that ID theorists 'would do well to reclaim the likes of Newton, Linnaeus, and Mendel as their own' (2007, p. 7). Fuller claims that Mendel was no evolutionist, but a 'special creationist with a grasp of probability theory'. For Fuller, the Mendelian rules of heredity are laws designed by God, that define 'the range of traits that God deemed permissible in a given species', but also enable

the development of new breeds and perhaps even species of plants and animals not included in the original divine plan (2006, p. 287).

The idea that Mendel was a 'special creationist' actually dates back to an article written in 1988 by L. A. Callender, called 'Gregor Mendel: An Opponent of Descent with Modification'. Callender sees Mendel as an advocate of the 'modified doctrine of Special Creation'2 — an idea which he traces back to Carolus Linnaeus.

Carolus Linnaeus
Carolus Linnaeus
(1707-1788)
Linnaeus once famously declared that there were only as many species in the world 'as the Infinite Being created in the beginning' (Species tot numeras, quot diversæ formæ in principio sunt creatæ). His own views changed, however; he later became convinced that species had evolved through hybridization. He conjectured that, at the Creation, God created ‘just so many different plants, as there are natural orders':
That He then so intermixed the plants of these orders by their marriages with each other, that as many plants were produced as there are now distinct genera. That Nature then intermixed these generic plants by reciprocal marriages … and multiplied them into all possible existing species… (cited Larson 1971, p.109)

Thus, for Linnaeus, even if God did not create all species at once, in the Creation, there was still a divine plan at work. This was true, even where species were ‘mixed with each other by Chance’, because 'the laws of the Creator' lead inexorably 'from the simple to the complex' (cited Larson 1971, p. 108). Thus, evolution occurs as part of a process of ‘continuous creation,’ within the framework of God’s laws.

Callender, then, claims Mendel as part of this Linnaean tradition of 'special creationists' who believe in descent through hybridisation (rather than modification). His revisionist account seems to be gaining currency, and not only among ID-ers. Now, when Mendel is categorised as a ‘Linnaean,’ it has become a form of code meaning ‘creationist’ (albeit of a ‘special’ kind). In Fabulous Science, for example, John Waller declares that Mendel 'adamantly rejected Darwinism in favour of the Linnaean approach' (2002, p. 154). After all, he was a man of God; and

what could better demonstrate the sublime skills of the Creator than evidence that a judicious, but modest, initial endowment of plant life had been given the in-built capacity to bring forth near infinite variety?' (p. 141).

Similarly, Jim Endersby writes that Mendel's

precise views are hard to ascertain, but it seems likely that he agreed with Linnaeus that hybridization could make new species by combining existing ones'—but only God could create the first plant forms (2007, p. 125).

Callender's ideas have even been taken up by Robert Olby (he who famously claimed that Mendel was 'no Mendelian'3). Olby suggests we should at least consider the possibility that Mendel was a 'Linnaean'. He writes:

How much have we been unconsciously allying Mendel with our own perspective because we want him on our side? Would it not have been reasonable for Mendel to have accepted some form of planned evolution, based on a law-following process the results of which could be predicted, and which God knew in advance? Of course, we will never know whether or not that was the case, but we should at least entertain the possibility when we try to interpret Mendel's work. (Olby, 1997)

There is a clear parallel between Olby's 'planned evolution' and Fuller's 'divine plan'.4 These are odd bedfellows, indeed; so what is happening here?

We may dismiss Fuller and his fellow ID-ers as a group on the lunatic fringe of science.5 However, when the revisionist version of Mendel as a ‘Linnaean’ (= ‘special creationist’) is promoted by a serious scholar such as Olby, it cannot so easily be ignored. So how valid are Olby's questions?


The 'man of God' fallacy

It is sometimes assumed that as a practising Catholic and 'man of God,' Mendel must have been a creationist. (For B. E. Bishop, he 'was a well-integrated member of his monastic community and a zealous defender of the faith, not a dissident'—1996, p. 205.) However, this is a fallacy. Mendel did not become a monk as the result of any strong religious faith. In 1843, he was an impecunious student, struggling to make ends meet; becoming a monk seemed the best way out for him. As he himself later wrote, he was obliged to enter a profession in which he would 'be spared perpetual anxiety about a means of livelihood' (Iltis 1932, p. 42).

Moreover, his monastery was no ordinary monastery. It was dedicated more to teaching and research than to prayer and meditation. In 1853, there was a formal inspection by a visiting bishop, who saw it as a hotbed of radical ideas and intellectual dissent. The monks' spiritual calling, the bishop claimed, was suffering as a result of all their teaching and scientific interests and commitments; he concluded that the monastery should be shut down. (It wasn't.) (See Orel 1996, p. 85.)

Finally, it has been suggested that Mendel's ideas were regarded as 'radical' in their time. The claustral prior at the monastery, A. Tkadlec, recalled that Mendel's discoveries 'did not meet with acclaim; indeed, he was even attacked and his theory suspected of being contrary to the revealed truths of the Christian religion' (cited Orel 1996, p. 195). Tkadlec, it is true, only became a member of the monastery in 1883, a year before Mendel died; and he wrote these lines in 1917, many years after the event. His evidence, then, may not be totally reliable; nevertheless, it reinforces the point that, in Mendel's case, we cannot assume any simple and automatic correlation between Catholicism and 'creationism.'

*

The only occasion that Mendel expressed himself directly on the subject of evolution was in an examination paper he sat in 1850. Discussing the origin of plant and animal forms, in the context of the formation of the earth, he wrote:

As soon as the earth in the course of time had achieved the necessary capability for the formation and maintenance of organic life, plants and animals of the lowest sorts first appeared.

In time, organic life

developed more and more abundantly; the oldest forms disappeared in part, to make space for new, more perfect ones.

This, Mendel wrote, is 'at the present time the generally accepted view of the emergence and development of the earth (Orel 1984, p. 237-8).6

It is far from any ‘creationism’ (even ‘special creationism’).

Mendel Pisum
Some of the Pisum variants
studied by Mendel

When he wrote this, he had been a monk for seven years. He finished his studies at the University of Vienna three years later, in 1853; began his Pisum experiments in earnest in 18567; and did not deliver his talk on those experiments until 1865. Of course, his ideas may have changed in time—and in any case, the views expressed in an examination may not always reflect the writer's real opinion; but in the absence of any other statement by Mendel on the origin of species, this would appear to undermine any notion that he was a 'special creationist.'

Vitĕslav Orel and Gerhard Czihak point out that Mendel was probably influenced, in preparing his exam paper, by Matthias Schleiden, who, in his book Die Pflanze und ihr Leben (which Mendel read), saw the origin of life in the simple cell. (See Orel 1984, pp. 229-230.) For Schleiden, the whole wealth of the plant kingdom may have been formed by gradual evolution from the cell, 'through varieties, sub-species and species, and thus onward', over a vast period of time (Schleiden 1853, p. 291). Schleiden also referred, however, to the notion of a Creator: for him, the plant kingdom 'is the rich altar-cloth of the temple of God' (p. 294). In the light of this, we may consider it significant that Mendel, in his examination essay, made no mention of supernatural agency, but only offered a naturalistic account of the origin of organic life. In fact, Mendel made no mention of God anywhere in his scientific writings; he scrupulously kept religion out of science, adhering strictly to a methodological naturalism. In a letter to Carl Nägeli, he called himself 'an empirical worker', whose theoretical conclusions were based on precise experiments that could be 'repeated and verified' (Stern and Sherwood 1966, p. 638).

It is pointless, in fact, to speculate that Mendel might have been a ‘Linnaean,’ or that he may have believed in some form of 'planned evolution' or design, partly because we simply have no evidence for this; but also because he himself effectively ruled metaphysical speculation out of the equation in scientific matters. Postulating that he might have been a 'special creationist' can only serve the interests of those creationists or ID-ers who wish to claim Mendel as one of their own.

Works Cited
Biology Dictionary >>
Famous Biologists >>


Entwicklungsgeschichte


In his Pisum paper, Mendel said that he saw his experiments as

the one correct way of finally reaching the solution to a question whose significance for the evolutionary history of organic forms must not be underestimated (OG, p. 2).

Daniel Fairbanks and Bryce Rytting seize on this and conclude:

Thus, Mendel was clearly interested in evolution and he considered his experiments as relevant to an understanding of evolution. (2001, p. 748)

Similarly, Ronald Fisher claimed that Mendel understood his laws to 'form a necessary basis for the understanding of the evolutionary process' (1936, p. 118). Bishop, however—the same Bishop who said Mendel was 'antievolutionary'—points out that Mendel did not use the word 'evolution', in the modern sense; even Darwin did not use the word at first. Rather, it only came into vogue as a synonym for 'natural selection' sometime after the publication of The Origin of Species.9 (See Bishop 1996, p. 205.)

In the English translation of the Pisum paper, the phrase ‘evolutionary history’ is a rendering of the German word Entwicklungsgeschichte, which is more accurately translated as 'history of development'. The term was applied, in particular, to embryology; but also, more generally, to the formation and development of organic entities. In his book, Grundzüge der wissenschaftlichen Botanik, Matthias Schleiden made Entwicklungsgeschichte the central principle of the science of botany. He urged botanists to study the laws of change and functional-morphological development in plants; to always see a plant, in its present form, as the product of past changes, but also as bearing within it the germs of its future development (see Schleiden 1842, p. 100).

When Mendel referred to Entwicklungsgeschichte, then, his audience would have understood this as a reference to the concept of development in botany, rather than Darwinian evolution as such. That is not the end of the story, however; and Bishop's confidence that the question of 'development' can be totally separated from the question of 'evolution' is misplaced. Mendel's tutor at the University of Vienna, Franz Unger, for example, explicitly linked Entwicklungsgeschichte to evolution. Only the Entwicklungsgeschichte of the entire plant world, he wrote, can explain the metamorphosis of species. (See Unger 1852, p. 122-3.)

Following the German publication of the Origin of Species, the concept of Entwicklungsgeschichte was rethought in Darwinian terms. Ernst Haeckel, in Generelle morphologie der Organismen (1866), pointed out that in the past, Entwicklungsgeschichte had been applied only to the development of the individual, and not to the evolution of phyla; but in fact (Haeckel maintained) these two branches of science—ontogeny and phylogeny—were inseparable. The Darwinian 'theory of descent alone is able to explain the history of the development of the organisms to us,' he concluded (1866, p. 7).10

The concept of Entwicklungsgeschichte, then, was ineluctably entangled with the question of evolution—and in particular, the whole 'species question.' The issue was not simply: how should species be defined?—but more importantly (as Albert Kölliker saw), whether they are ‘to be regarded in their entirety as fixed and constant creations, or have the ability to change’ and evolve into other species (1879, p. 4).


Kölreuter and Gärtner

In Mendel’s time, then, the ‘species debate’ was the ‘evolution debate.’ For the advocates of species fixity, plant hybridisation posed a particular problem. The Linnaean doctrine, Species tot numeras, quot diversæ formæ in principio sunt creatæ, was an article of faith for many botanists. Understandably, then, when Linnaeus himself moved away from the doctrine of species fixity and proposed his theory of descent through hybridisation, it quickly came under attack. His contemporary, Michel Adanson, argued that the supposed 'new hybrid species' that Linnaeus (and others) claimed to have discovered were in fact only mutants—the 'transmutation of species' had not taken place. Species can only vary within limits; in the end, 'they return into the harmonious order re-established by the wisdom of the Creator' (cited Glass 1968, p. 156).

Joseph Kölreuter tested some of the hybrids that Linnaeus had produced; and disputed that they were new species. He decried Linnaeus's views on hybridisation as 'adventurous and going against all reason’; and feared the ‘astonishing confusion’, the ‘monstrous swarm of imperfections’ that would be the consequence of plants ‘unchangingly and constantly conserving their bastard species’ (Müller-Wille and Orel 2007, pp. 181-3). Kölreuter maintained, however, that the great Creator had laid down laws and mechanisms to prevent such disorder: hybrids between species were always sterile, while hybrids between varieties were always subject to reversion and transformation. (Reversion refers to the idea that, when self-fertilised, hybrids tend to revert to one or other of the original parental forms; if they are cross-fertilised with one of the parent plants, they are 'transformed' in time, becoming more and more like the parental form with each new generation.)

The most famous (pre-Darwinian) hybridist after Kölreuter is Carl Freidrich von Gärtner. In Versuche und Beobachtungen über die Bastarderzeugung im Pflanzenreich (1849), Gärtner reported the results of nearly 10,000 crossing experiments. He declared that his goal was to repudiate the idea that hybridisation could be 'the source of the wealth of species' (1849, p. 153). He insisted that nature tends 'to preserve species, and to permit no disorder in reproduction, and to set a natural limit to the same through infertility' (p. 555). This, he said, should reassure those 'friends of the plant realm' who feared that hybridisation was an interference in the plan of the Creator (p. 556).11

Both Gärtner and Kölreuter, then, were traditional creationists. The species for them was the unit of creation, part of the divine order and design of the universe. Gärtner placed the species debate in the wider context of the evolution debate: he attacked pre-Darwinian evolutionists such as Gottfried Reinhold Treviranus, or Jean Baptiste d'Omalius d'Halloy, and questioned their evidence for transmutation. He pointed to evidence that supported species stability; for example, he claimed that:

  • species change was not proven
  • numerous plant forms had not noticeably changed over thousands of years
  • the geological record confirmed the stability of plant species
  • it was a law of nature that varieties, under given circumstances, reverted automatically to the species prototype. (See VB, pp. 157-9)

(It is slightly unnerving, in this book written in 1849, to find Gärtner using many of the same arguments against evolution that are still used by creationists and ID-ers today.)

Works Cited
Biology Dictionary >>
Famous Biologists >>


Attempts to define ‘species’

For proponents of species fixity, it was vital to maintain a distinction between species and varieties, or the theory would collapse (leaving only Kölreuter's ‘astonishing confusion’ of forms). Hybrids between varieties might exist; but hybrids between species would break the immutable species barrier; a species would be just a more developed variety. However, a variety of nineteenth century naturalists (Darwin among them) denied that there was any real boundary-line between variety and species—a point that was echoed by Mendel in his Pisum paper. It is impossible, he wrote, 'to draw a sharp line between species and varieties', or to establish any 'fundamental difference between the hybrids of species and those of varieties' (OG, p. 5).

Mendel noted the difficulties in agreeing on any systematic classification of species. According to the 'strictest definition … only those individuals that display identical traits under identical conditions belong to a species'; but, for Mendel, this would mean that, in fact, no two individuals 'could be counted as one and the same species' (OG, p. 5). Schleiden saw that 'identical conditions' were in any case impossible to achieve. 'Even if we maintain the importance of the history of development [Entwicklungsgeschichte] as the principle of botany,' he wrote, 'then we can only look for the concept of the plant species in the way that, over a period of time [i.e. several generations], a certain group of traits proves itself constant and identical' (cited Haeckel 1866, pp. 331). This, indeed, was the principle which Mendel followed: before beginning his Pisum experiments, he observed specimens of pea-plant over a two-year period, to ensure they bred true, i.e. a certain group of traits (wrinkled/smooth, green/yellow, etc.) proved to be constant. Subsequently, he looked for the constancy of morphological traits in the progeny of the hybrids he produced, through several generations.12

Carl Wilhelm von Nageli
Carl Nageli
(1817–1891)

The botanist Carl Nägeli also saw no fixed boundary between species and varieties. He regretted that the 'most numerous and important investigations into hybrid formation' had been carried out by proponents of species fixity. If, nevertheless, their investigations showed that the distinction between the hybrids of varieties, and the hybrids of species, did not exist, then the results were all the more significant (1866, p. 18813). Gärtner had, in fact, found species-hybrids in several genera such as Dianthus, Geum, Gladiolus and Crinum which he said 'remained constant, and have not yielded any varieties up to the sixth and eighth generations' (VB, p. 149). Dianthus hybrids continued 'up to the tenth generation, without change in type'; nevertheless, he argued that such hybrids showed, in the end, a decrease in fertility, and were doomed to die out, thus confirming that there are 'natural limits' on hybridisation; even fertile hybrids could not become stabilized as species (VB, pp. 553-5). But Nägeli did not accept Gärtner's evidence here. Perhaps the conditions in Gärtner's experiments were simply unfavourable for promoting fertility, he suggested. (If you handled 'pure species' in the same way, he said, the odds were 100-1 that they would die out too.) If, despite this,

the fertility in individual species hybrids increased through several generations, then we must assume that they are not inferior in fertility to pure species, and that they can arrive at a duration and a constancy which is equal to that of species (BM, p. 207).

(Darwin similarly thought there were problems with Gärtner's methods, which may have caused sterility in his specimens—see Darwin 1859, p. 247.)

Like Nägeli, Mendel cited Gärtner's experiments, not as evidence of species fixity, but the opposite—the existence of constant hybrids. He wrote:

Gärtner had an opportunity of following the Dianthus Armeria-deltoides to its tenth generation, since that plant propagated itself regularly in the garden (OG, p. 41).

Significantly, Mendel failed to mention Gärtner's claim that he saw a steady decrease in the fertility of the Dianthus armeria-deltoides hybrid. Callender even accuses him of being economical with the truth (see LAC, p. 57). It seems that—like Nägeli and Darwin—Mendel questioned both the reliability of Gärtner's results, and the conclusions he drew from them. (he also cited Max Wichura's Salix hybrids as examples of constant hybrids; but Wichura in fact claimed that hybrids of this species are only able to self-fertilise 'for a certain number of generations with continual diminution of their sexual potency and vigour' [cited Olby 1985, p. 287]. Again, Mendel failed to mention this.)


Transformation

In a 'fragment' of the Pisum paper that has been much debated, Mendel wrote:

The success of transformation experiments led Gärtner to disagree with those scientists who contest the stability of plant species and assume continuous evolution of plant forms. In the complete transformation of one species into another he finds unequivocal proof that a species has fixed limits beyond which it cannot change. Although this opinion cannot be adjudged unconditionally valid, considerable confirmation of the earlier expressed conjecture on the variability of cultivated plants is to be found in the experiments performed by Gärtner. (OG, p. 47)

Gavin de Beer claimed that this passage 'comes as near to the acceptance of the mutability of species as anyone could wish' (cited LAC, p. 55). However, Callender contends that, if Mendel's statement is taken literally, as he

most assuredly intended it to be taken, then it says quite simply that he gave conditional acceptance to the view, expressed by Gaertner [sic], "that species are fixed within limits beyond which they cannot change". Nothing could be clearer. (LAC, p. 54; emphasis in original.)

The key phrase here, in the Pisum paper, is this: 'Wenn auch dieser Ansicht eine bedingungslose Geltung nicht zuerkannt werden kann', meaning literally, 'Even though this opinion cannot be granted an unconditional validity' (or, 'Although this opinion cannot be adjudged unconditionally valid'). Callender, however, uses William Bateson's 1909 translation: 'Although this opinion cannot be unconditionally accepted…' (Bateson 1913, p. 379). The difference is slight but significant. One version (Bateson's) suggests accepting or rejecting Gärtner's argument is a matter of personal judgement; the other (more accurate) translation suggests a more objective, scientific process of assessing an argument's 'validity'. Opinion, it is implied, must be tested empirically. (In fact, when Mendel used the term Geltung [validity], it was always in connection with scientific testability: for example, he referred to the 'value and validity' [Werth und Geltung] of an experiment as determined by the means used; and he stressed the need to test the 'validity' [Geltung] of his own Pisum laws as they applied to other plants.)

Callender goes on to paraphrase Bateson's translation, and adds his own small but significant linguistic twist. Whereas, in Bateson, Mendel says that Gärtner's views 'cannot be unconditionally accepted', Callender says he 'conditionally accepted' them—a much more emphatic statement. In this way, Callender turns Mendel into an advocate of species fixity. But his case rests on just this: a semantic shift on a faulty translation.

Wolf-Ekkehard Lönnig (a notorious proponent of Intelligent Design14) takes things a step further. He claims the whole thrust of Mendel's work was to prove Gärtner's claim that species are, indeed, fixed within certain limits. (See Lönnig 1998, 2000, 2001.) However, in that case, it makes little sense that Mendel would put aside Gärtner's claims that hybrids cannot become stable species and are doomed by the laws of nature to degenerate and become infertile.

We should note that in this, the concluding section of his paper, Mendel is only assessing the 'validity' of one aspect of Gärtner's case for species fixity—his 'transformation' experiments. (He does not refer at this point to the issue of hybrid infertility at all.) He is not, then (as Callender and Lönnig assume) making a general statement in support of species fixity—or, for that matter, in support of species mutability. We can only say that Mendel gives conditional acceptance—or rather, he does not wholly reject—Gärtner's conclusions on the 'limits' of species change, as far as they relate to the particular issue of transformation.

In his back-crossing experiments, Gärtner (like Kölreuter before him) had transformed species 'A' into species 'B' by producing a hybrid between them, and then using pollen from species 'B' to fertilise these hybrids. This was repeated over several succeeding generations, until a form was arrived at which was, as Mendel noted, like 'B' in appearance, and 'constant in its progeny' (OG, p. 44). For Gärtner, this contradicted the idea that a plant, once it has been 'diverted' from its true nature, could never go back to its original form; and for him, this showed again that there are firm limits set on a plant species, 'beyond which it cannot change, but must return to the prototype, or cease to exist' (VB, p. 475). So the choice was: revert to parental type—or die. For Gärtner, transformation (and reversion) were clear evidence for species stability, and against evolution.

Works Cited
Biology Dictionary >>
Famous Biologists >>

Mendel provided his own explanation for the process of transformation—one which did not depend on some force in the 'innermost nature' of the plant, as Gärtner claimed (VB, p. 605); but was, in fact, consistent with the Pisum laws. He saw that, with repeated backcrossing of hybrids to only one parental form, those 'factors' (as he called them; we would term them genes) which differ between the two parental types, are gradually replaced with ones derived from the backcross parent. The length of time and number of fertilisations it takes to wholly transform species 'A' into species 'B' depends on the number of experimental plants, and the number of differing traits in the parental species (OG, p. 45).

In this way, transformation can be explained—but it does not, as Mendel recognised, offer 'unconditional proof' that a species has fixed limits beyond which it cannot change.

In fact, he distinguished between 'variable' and 'constant' hybrids. He saw the Pisum laws as applicable only to other, variable hybrids; different laws, he said, would govern the production of constant hybrids. Variable hybrids (like peas) produced different kinds of germinal and pollen cells (i.e. the genetic factors segregated); they were therefore varied in their progeny. The progeny of constant hybrids, however, remained constant. This was significant for the developmental history of plants [Entwicklungsgeschichte], 'because constant hybrids attain the status of new species' (OG, p. 41; emphasis in original).

We should note, then, that constant hybrids would resist transformation to parental type. If constant hybrids could be shown to exist, then Gärtner's laws of reversion and transformation would be broken. There would be no firm limits beyond which a species could not change.

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